feat(cli): sunbeam vpn create-key + vpn-tls-insecure config flag

`sunbeam vpn create-key` calls Headscale's REST API at
`/api/v1/preauthkey` to mint a new pre-auth key for onboarding a new
client. Reads `vpn-url` and `vpn-api-key` from the active context;
the user generates the API key once via `headscale apikeys create`
on the cluster and stores it in their context config.

Flags:
- --user <name>      Headscale user the key belongs to
- --reusable         allow multiple registrations with the same key
- --ephemeral        auto-delete the node when its map stream drops
- --expiration <dur> human-friendly lifetime ("30d", "1h", "2w")

Also adds a `vpn-tls-insecure` context flag that controls TLS
verification across the whole VPN integration: it's now used by both
the daemon (for the Noise control connection + DERP relay) and the
new create-key REST client. Test stacks with self-signed certs set
this to true; production stacks leave it false.

Verified end-to-end against the docker test stack:

  $ sunbeam vpn create-key --user test --reusable --expiration 1h
  ==> Creating pre-auth key on https://localhost:8443
      Pre-auth key for user 'test':
  ebcd77f51bf30ef373c9070382b834859935797a90c2647f

  Add it to a context with:
    sunbeam config set --context <ctx> vpn-auth-key ebcd77f5...

src/cli.rs

@@ -206,6 +206,22 @@ pub enum Verb {
 pub enum VpnAction {
     /// Show VPN tunnel status.
     Status,
+    /// Create a new pre-auth key for onboarding a new client.
+    CreateKey {
+        /// Headscale user the key belongs to (default: from CLI user).
+        #[arg(long, default_value = "sunbeam")]
+        user: String,
+        /// Make the key reusable across multiple registrations.
+        #[arg(long)]
+        reusable: bool,
+        /// Mark the key (and any node registered with it) ephemeral —
+        /// Headscale auto-deletes the node when its map stream drops.
+        #[arg(long)]
+        ephemeral: bool,
+        /// Key lifetime, in human-readable form (e.g. "1h", "30d").
+        #[arg(long, default_value = "30d")]
+        expiration: String,
+    },
 }
 
 #[derive(Subcommand, Debug)]
@@ -1532,6 +1548,12 @@ pub async fn dispatch() -> Result<()> {
 
         Some(Verb::Vpn { action }) => match action {
             VpnAction::Status => crate::vpn_cmds::cmd_vpn_status().await,
+            VpnAction::CreateKey {
+                user,
+                reusable,
+                ephemeral,
+                expiration,
+            } => crate::vpn_cmds::cmd_vpn_create_key(&user, reusable, ephemeral, &expiration).await,
         },
 
         Some(Verb::VpnDaemon) => crate::vpn_cmds::cmd_vpn_daemon().await,

src/config.rs

@@ -72,6 +72,23 @@ pub struct Context {
     /// empty, falls back to a static fallback address.
     #[serde(default, rename = "vpn-cluster-host", skip_serializing_if = "String::is_empty")]
     pub vpn_cluster_host: String,
+
+    /// Headscale API key for `sunbeam vpn create-key` and other admin
+    /// commands. Generated once via `headscale apikeys create`. Stored
+    /// in plain text — keep this file readable only by the user.
+    #[serde(default, rename = "vpn-api-key", skip_serializing_if = "String::is_empty")]
+    pub vpn_api_key: String,
+
+    /// Skip TLS certificate verification when talking to the VPN
+    /// coordination server (control plane, DERP relay, REST API).
+    /// Only set this for test stacks with self-signed certs — leave
+    /// false for production.
+    #[serde(default, rename = "vpn-tls-insecure", skip_serializing_if = "is_false")]
+    pub vpn_tls_insecure: bool,
+}
+
+fn is_false(b: &bool) -> bool {
+    !*b
 }
 
 // ---------------------------------------------------------------------------

src/vpn_cmds.rs

@@ -187,8 +187,7 @@ async fn run_daemon_foreground() -> Result<()> {
         control_socket: state_dir.join("daemon.sock"),
         hostname,
         server_public_key: None,
-        // Production deployments use proper TLS — leave verification on.
-        derp_tls_insecure: false,
+        derp_tls_insecure: ctx.vpn_tls_insecure,
     };
 
     step(&format!("Connecting to {}", ctx.vpn_url));
@@ -279,6 +278,175 @@ pub async fn cmd_vpn_status() -> Result<()> {
     Ok(())
 }
 
+/// Run `sunbeam vpn create-key` — call Headscale's REST API to mint a
+/// new pre-auth key for onboarding a new client.
+///
+/// Reads `vpn-url` and `vpn-api-key` from the active context. The user
+/// must have generated a Headscale API key out-of-band (typically via
+/// `headscale apikeys create` on the cluster) and stored it in the
+/// context config.
+pub async fn cmd_vpn_create_key(
+    user: &str,
+    reusable: bool,
+    ephemeral: bool,
+    expiration: &str,
+) -> Result<()> {
+    let ctx = active_context();
+    if ctx.vpn_url.is_empty() {
+        return Err(SunbeamError::Other(
+            "no vpn-url configured for this context".into(),
+        ));
+    }
+    if ctx.vpn_api_key.is_empty() {
+        return Err(SunbeamError::Other(
+            "no vpn-api-key configured — generate one with \
+             `kubectl exec -n vpn deploy/headscale -- headscale apikeys create` \
+             and add it to your context as vpn-api-key"
+                .into(),
+        ));
+    }
+
+    // Headscale's REST API mirrors its gRPC schema. Body fields use
+    // snake_case in the JSON request.
+    let body = serde_json::json!({
+        "user": user,
+        "reusable": reusable,
+        "ephemeral": ephemeral,
+        "expiration": expiration_to_rfc3339(expiration)?,
+    });
+
+    let endpoint = format!("{}/api/v1/preauthkey", ctx.vpn_url.trim_end_matches('/'));
+    let client = reqwest::Client::builder()
+        .danger_accept_invalid_certs(ctx.vpn_tls_insecure)
+        .build()
+        .map_err(|e| SunbeamError::Other(format!("build http client: {e}")))?;
+
+    step(&format!("Creating pre-auth key on {}", ctx.vpn_url));
+    let resp = client
+        .post(&endpoint)
+        .bearer_auth(&ctx.vpn_api_key)
+        .json(&body)
+        .send()
+        .await
+        .map_err(|e| SunbeamError::Other(format!("POST {endpoint}: {e}")))?;
+
+    let status = resp.status();
+    let text = resp
+        .text()
+        .await
+        .map_err(|e| SunbeamError::Other(format!("read response body: {e}")))?;
+
+    if !status.is_success() {
+        return Err(SunbeamError::Other(format!(
+            "headscale returned {status}: {text}"
+        )));
+    }
+
+    // Response shape: {"preAuthKey": {"key": "...", "user": "...", "reusable": ..., ...}}
+    let parsed: serde_json::Value = serde_json::from_str(&text).map_err(|e| {
+        SunbeamError::Other(format!("parse headscale response: {e}\nbody: {text}"))
+    })?;
+    let key = parsed
+        .get("preAuthKey")
+        .and_then(|p| p.get("key"))
+        .and_then(|k| k.as_str())
+        .ok_or_else(|| {
+            SunbeamError::Other(format!("no preAuthKey.key in response: {text}"))
+        })?;
+
+    ok(&format!("Pre-auth key for user '{user}':"));
+    println!("{key}");
+    println!();
+    println!("Add it to a context with:");
+    println!("  sunbeam config set --context <ctx> vpn-auth-key {key}");
+    Ok(())
+}
+
+/// Convert a human-friendly duration ("30d", "1h", "2w") into the RFC3339
+/// timestamp Headscale expects in pre-auth key requests.
+fn expiration_to_rfc3339(s: &str) -> Result<String> {
+    let s = s.trim();
+    if s.is_empty() {
+        return Err(SunbeamError::Other("empty expiration".into()));
+    }
+    let (num, unit) = s.split_at(s.len() - 1);
+    let n: u64 = num
+        .parse()
+        .map_err(|_| SunbeamError::Other(format!("bad expiration '{s}': expected like '30d'")))?;
+    let secs = match unit {
+        "s" => n,
+        "m" => n * 60,
+        "h" => n * 3600,
+        "d" => n * 86_400,
+        "w" => n * 86_400 * 7,
+        other => {
+            return Err(SunbeamError::Other(format!(
+                "bad expiration unit '{other}': expected s/m/h/d/w"
+            )));
+        }
+    };
+    let when = std::time::SystemTime::now()
+        .checked_add(std::time::Duration::from_secs(secs))
+        .ok_or_else(|| SunbeamError::Other("expiration overflow".into()))?
+        .duration_since(std::time::UNIX_EPOCH)
+        .map_err(|e| SunbeamError::Other(format!("system time: {e}")))?
+        .as_secs();
+    // Format as RFC3339 manually using the same proleptic Gregorian
+    // approach as elsewhere in this crate. Headscale parses Go's
+    // time.RFC3339, which is `YYYY-MM-DDTHH:MM:SSZ`.
+    Ok(unix_to_rfc3339(when as i64))
+}
+
+/// Convert a unix timestamp (seconds since epoch) to an RFC3339 string.
+fn unix_to_rfc3339(secs: i64) -> String {
+    let days = secs.div_euclid(86_400);
+    let day_secs = secs.rem_euclid(86_400);
+    let h = day_secs / 3600;
+    let m = (day_secs % 3600) / 60;
+    let s = day_secs % 60;
+    let (year, month, day) = days_to_ymd(days);
+    format!("{year:04}-{month:02}-{day:02}T{h:02}:{m:02}:{s:02}Z")
+}
+
+/// Days since 1970-01-01 → (year, month, day). Proleptic Gregorian.
+fn days_to_ymd(mut days: i64) -> (i32, u32, u32) {
+    let mut y: i32 = 1970;
+    loop {
+        let leap = (y % 4 == 0 && y % 100 != 0) || y % 400 == 0;
+        let year_days: i64 = if leap { 366 } else { 365 };
+        if days < year_days {
+            break;
+        }
+        days -= year_days;
+        y += 1;
+    }
+    let leap = (y % 4 == 0 && y % 100 != 0) || y % 400 == 0;
+    let mdays = [
+        31u32,
+        if leap { 29 } else { 28 },
+        31,
+        30,
+        31,
+        30,
+        31,
+        31,
+        30,
+        31,
+        30,
+        31,
+    ];
+    let mut mo = 1u32;
+    let mut days_u = days as u32;
+    for md in mdays {
+        if days_u < md {
+            break;
+        }
+        days_u -= md;
+        mo += 1;
+    }
+    (y, mo, days_u + 1)
+}
+
 /// Resolve the on-disk directory for VPN state (keys, control socket).
 fn vpn_state_dir() -> Result<PathBuf> {
     let home = std::env::var("HOME")