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

A request past the server's 30-second limit answered 408, which tells the
client it was too slow sending its request, and /api/v1/stats answered 500
when its 4-second read ran out. The limit is now chi's middleware.Timeout,
which answers 504 and never writes from a second goroutine while the
handler is writing.

Right after each database read or host name lookup, whatever came back, a
handler asks deadlinePassed whether its time limit has passed and, if so,
answers 504, with the JSON error body on JSON routes: a cut-off read can
return part of its rows without an error. The health check is unchanged.

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())