feat(cache): add pingora-cache integration with per-route config

Add in-memory HTTP response cache using pingora-cache MemCache backend.
Cache runs after the detection pipeline so cache hits bypass upstream
request modifications and body rewriting. Respects Cache-Control
(no-store, private, s-maxage, max-age), skips caching for routes with
body rewrites or auth subrequest headers, and supports configurable
default TTL, stale-while-revalidate, and max file size per route.

Signed-off-by: Sienna Meridian Satterwhite <sienna@sunbeam.pt>

src/config.rs

@@ -190,6 +190,25 @@ pub struct HeaderRule {
     pub value: String,
 }
 
+/// Per-route HTTP response cache configuration.
+#[derive(Debug, Deserialize, Clone)]
+pub struct CacheConfig {
+    #[serde(default = "default_cache_enabled")]
+    pub enabled: bool,
+    /// Default TTL in seconds when the upstream response has no Cache-Control header.
+    #[serde(default = "default_cache_ttl")]
+    pub default_ttl_secs: u64,
+    /// Seconds to serve stale content while revalidating in the background.
+    #[serde(default)]
+    pub stale_while_revalidate_secs: u32,
+    /// Max cacheable response body size in bytes (0 = no limit).
+    #[serde(default)]
+    pub max_file_size: usize,
+}
+
+fn default_cache_enabled() -> bool { true }
+fn default_cache_ttl() -> u64 { 60 }
+
 #[derive(Debug, Deserialize, Clone)]
 pub struct RouteConfig {
     pub host_prefix: String,
@@ -220,6 +239,9 @@ pub struct RouteConfig {
     /// Extra response headers added to every response for this route.
     #[serde(default)]
     pub response_headers: Vec<HeaderRule>,
+    /// HTTP response cache configuration for this route.
+    #[serde(default)]
+    pub cache: Option<CacheConfig>,
 }
 
 impl Config {