feat(backend): server hardening — timeouts, security headers, trusted-proxy client IP (closes #19)

Add ReadHeaderTimeout and IdleTimeout to the http.Server (named
constants beside the existing timeouts) to close the slowloris and
idle-keep-alive gaps.

Add a SecurityHeaders middleware setting HSTS, a JSON-API CSP
(default-src 'none'; frame-ancestors 'none'), X-Frame-Options: DENY,
X-Content-Type-Options: nosniff, Referrer-Policy, and Permissions-Policy.
It is registered before CORS so the headers ride on preflight responses.

Resolve the client IP from X-Forwarded-For / X-Real-IP only when the
direct peer is in a trusted-proxy allowlist, defaulting to loopback plus
RFC1918 and configurable via TRUSTED_PROXIES; an untrusted peer's
forwarded headers are ignored and the direct peer is logged. Uses
net/netip; no new dependency.

Model: opus-4-8

backend/internal/server/http.go

@@ -8,20 +8,24 @@ import (
 )
 
 const (
-	readTimeout    = 10 * time.Second
-	writeTimeout   = 10 * time.Second
-	maxHeaderBytes = 1 << 20 // 1 MiB
+	readTimeout       = 10 * time.Second
+	readHeaderTimeout = 5 * time.Second
+	writeTimeout      = 10 * time.Second
+	idleTimeout       = 60 * time.Second
+	maxHeaderBytes    = 1 << 20 // 1 MiB
 )
 
 func (s *Server) serveUntilShutdown() {
 	listenAddr := fmt.Sprintf(":%d", s.params.Config.Port)
 
 	s.httpServer = &http.Server{
-		Addr:           listenAddr,
-		Handler:        s,
-		MaxHeaderBytes: maxHeaderBytes,
-		ReadTimeout:    readTimeout,
-		WriteTimeout:   writeTimeout,
+		Addr:              listenAddr,
+		Handler:           s,
+		MaxHeaderBytes:    maxHeaderBytes,
+		ReadTimeout:       readTimeout,
+		ReadHeaderTimeout: readHeaderTimeout,
+		WriteTimeout:      writeTimeout,
+		IdleTimeout:       idleTimeout,
 	}
 
 	s.SetupRoutes()