fix(db): fast-fail Indexer::init on non-SQLite files via magic-header preflight

tests/dynamic_fixtures/lang_detect/cli_python

@@ -0,0 +1,10 @@
+#!/usr/bin/env python3
+# Extensionless CLI entry point. Shebang-only language identification.
+import os
+import sys
+
+def handle_request(payload: str) -> None:
+    os.system("echo " + payload)
+
+if __name__ == "__main__":
+    handle_request(sys.argv[1])