Answer 504, not 408, when a request outlasts its time limit (closes #70)

The 30-second limit is now chi's middleware.Timeout, which answers 504
and never writes from a second goroutine. It runs after
JSONResponseMiddleware, so a handler that wrote nothing is not sent a
200 first.

One rule covers the handlers: right after each database read or host
name lookup, before looking at what came back, error or not, a handler
asks deadlinePassed whether the time limit has passed and, if so,
answers 504, with the JSON error body on JSON routes. A cut-off read can
return an error, or part of its rows or none without one, so only this
check tells it from a complete read. JSONValidationMiddleware answers
504 with the JSON error body when the limit has passed and the handler
wrote nothing.

Model: opus-5-5

internal/server/routes.go

@@ -16,9 +16,12 @@ func (s *Server) setupRoutes() {
 	r.Use(middleware.RealIP)
 	r.Use(RequestLoggerMiddleware(s.logger.Logger)) // Structured request logging
 	r.Use(middleware.Recoverer)
-	const requestTimeout = 30 * time.Second // Increased from 8s for slow queries
-	r.Use(TimeoutMiddleware(requestTimeout, s.logger.Logger))
 	r.Use(JSONResponseMiddleware)
+	// Timeout must come after JSONResponseMiddleware, which sends its answer
+	// when the handler returns and would send a 200 ahead of Timeout's 504 for
+	// a handler that wrote nothing.
+	const requestTimeout = 30 * time.Second // Increased from 8s for slow queries
+	r.Use(middleware.Timeout(requestTimeout))
 
 	// Routes
 	r.Get("/", s.handleIndex())