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/db/migrations.go

@@ -96,7 +96,7 @@ func migrate001CreateCoreTables(tx *sql.Tx) error {
 func migrate002AddL7Fields(tx *sql.Tx) error {
 	stmts := []string{
 		`ALTER TABLE listeners ADD COLUMN proxy_protocol INTEGER NOT NULL DEFAULT 0`,
-		`ALTER TABLE routes ADD COLUMN mode TEXT NOT NULL DEFAULT 'l4'`,
+		`ALTER TABLE routes ADD COLUMN mode TEXT NOT NULL DEFAULT 'l4' CHECK(mode IN ('l4', 'l7'))`,
 		`ALTER TABLE routes ADD COLUMN tls_cert TEXT NOT NULL DEFAULT ''`,
 		`ALTER TABLE routes ADD COLUMN tls_key TEXT NOT NULL DEFAULT ''`,
 		`ALTER TABLE routes ADD COLUMN backend_tls INTEGER NOT NULL DEFAULT 0`,