Fix server threading, tag URLs, and tag date resolution

- Switch to ThreadingHTTPServer to handle concurrent requests
- Catch BrokenPipeError on response writes to suppress noisy tracebacks
- Change tag URL from /tags/ to /releases/tag/
- Resolve tag dates via commit SHA from tags API + commits endpoint (works for public repos without auth)
- Update tests to match new tag date resolution approach

src/cli.py

@@ -307,11 +307,11 @@ def cmd_serve(args: argparse.Namespace, db_path: str) -> None:
     port = args.port or DEFAULT_SERVER_PORT
 
     from src.server import create_handler
-    from http.server import HTTPServer
+    from http.server import ThreadingHTTPServer
 
     handler = create_handler(db_path)
     try:
-        server = HTTPServer((host, port), handler)
+        server = ThreadingHTTPServer((host, port), handler)
     except OSError as e:
         if e.errno == 98:  # Address already in use
             error(f"Port {port} is already in use")