Fix three doc-vs-implementation gaps found during audit

1. DB migration: add CHECK(mode IN ('l4', 'l7')) constraint on the
   routes.mode column. ARCHITECTURE.md documented this constraint but
   migration v2 omitted it. Enforces mode validity at the database
   level in addition to application-level validation.

2. L7 reverse proxy: distinguish timeout errors from connection errors
   in the ErrorHandler. Backend timeouts now return HTTP 504 Gateway
   Timeout instead of 502. Uses errors.Is(context.DeadlineExceeded)
   and net.Error.Timeout() detection. Added isTimeoutError unit tests.

3. Config validation: warn when L4 routes have tls_cert or tls_key set
   (they are silently ignored). ARCHITECTURE.md documented this warning
   but config.validate() did not emit it. Uses slog.Warn.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/config/config.go

@@ -3,6 +3,7 @@ package config
 import (
 	"crypto/tls"
 	"fmt"
+	"log/slog"
 	"os"
 	"strings"
 	"time"
@@ -198,6 +199,12 @@ func (c *Config) validate() error {
 					i, l.Addr, j, r.Hostname, r.Mode)
 			}
 
+			// Warn if L4 routes have cert/key set (they are ignored).
+			if r.Mode == "l4" && (r.TLSCert != "" || r.TLSKey != "") {
+				slog.Warn("L4 route has tls_cert/tls_key set (ignored)",
+					"listener", l.Addr, "hostname", r.Hostname)
+			}
+
 			// L7 routes require TLS cert and key.
 			if r.Mode == "l7" {
 				if r.TLSCert == "" || r.TLSKey == "" {