Add the HTTP hardening the policy requires before 1.0 (closes #108)

Every answer carries the six security headers REPO_POLICIES.md lists.
The Content-Security-Policy allows inline content only with a nonce made
for that answer; the pages put it on their style and script blocks and
drop their four style attributes.

The request log and /ip/ take a forwarded address only from a proxy
listed in TRUSTED_PROXIES, which stops the start when set but invalid:
the last X-Forwarded-For entry that is not a trusted proxy. X-Real-IP
and True-Client-IP are no longer read.

The JSON routes and the health check send fixed messages in place of
database, parser and resolver errors, and log the detail.

Model: opus-5-5

internal/server/routes.go

@@ -13,8 +13,9 @@ func (s *Server) setupRoutes() {
 
 	// Middleware
 	r.Use(DecodedPathMiddleware)
+	r.Use(SecurityHeadersMiddleware)
 	r.Use(middleware.RequestID)
-	r.Use(middleware.RealIP)
+	r.Use(RealIPMiddleware(s.trustedProxies))
 	r.Use(RequestLoggerMiddleware(s.logger.Logger)) // Structured request logging
 	r.Use(middleware.Recoverer)
 	r.Use(JSONResponseMiddleware)