feat(static_files): add static file serving, SPA fallback, rewrites, body rewriting, and auth subrequests

Add static file serving with try_files chain ($uri, $uri.html,
$uri/index.html, fallback), regex-based URL rewrites compiled at
startup, response body find/replace for text/html and JS content,
auth subrequests with header capture for path routes, and custom
response headers per route. Extends RouteConfig with static_root,
fallback, rewrites, body_rewrites, and response_headers fields.

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

Cargo.toml

@@ -55,6 +55,13 @@ prometheus  = "0.13"
 # Request IDs
 uuid = { version = "1", features = ["v4"] }
 
+# Static file serving and body rewriting
+bytes = "1"
+regex = "1"
+
+# Auth subrequests
+reqwest = { version = "0.12", features = ["rustls-tls"], default-features = false }
+
 # Rustls crypto provider — must be installed before any TLS init
 rustls      = { version = "0.23", features = ["aws-lc-rs"] }