fix: bump Fiber ReadBufferSize to 16 KB

fasthttp defaults to a 4 KB read buffer per connection. Any request
whose header line exceeds that returns a flat HTTP 431 from Fiber
before the request reaches a handler — affecting clients carrying
chunked NextAuth cookies, mTLS client-cert headers, or large bearer
tokens.

16 KB matches the cluster ingress-nginx large_client_header_buffers
allowance. Tested 4–8 KB header payloads through shell-api.gosec.internal
— all return normal app responses instead of 431.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/server/main.go

@@ -44,6 +44,12 @@ func main() {
 		DisableStartupMessage: true,
 		ReadTimeout:           10 * time.Second,
 		WriteTimeout:          10 * time.Second,
+		// fasthttp defaults to a 4 KB read buffer per connection;
+		// any request whose header line exceeds that returns a flat
+		// HTTP 431 before any handler runs. 16 KB matches the
+		// cluster's nginx-ingress large_client_header buffer and
+		// accommodates chunked NextAuth cookies + large bearer tokens.
+		ReadBufferSize: 16384,
 	})
 	app.Use(recover.New())
 	app.Use(logger.New(logger.Config{