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/server.py

@@ -39,20 +39,29 @@ def create_handler(db_path: str):
             self.send_response(200)
             self.send_header("Content-Type", "application/rss+xml; charset=utf-8")
             self.end_headers()
-            self.wfile.write(feed_xml.encode("utf-8"))
+            try:
+                self.wfile.write(feed_xml.encode("utf-8"))
+            except BrokenPipeError:
+                pass
 
         def _serve_index(self):
             html = generate_index_html(self._base_url())
             self.send_response(200)
             self.send_header("Content-Type", "text/html; charset=utf-8")
             self.end_headers()
-            self.wfile.write(html.encode("utf-8"))
+            try:
+                self.wfile.write(html.encode("utf-8"))
+            except BrokenPipeError:
+                pass
 
         def _serve_health(self):
             self.send_response(200)
             self.send_header("Content-Type", "text/plain")
             self.end_headers()
-            self.wfile.write(b"OK")
+            try:
+                self.wfile.write(b"OK")
+            except BrokenPipeError:
+                pass
 
         def _base_url(self) -> str:
             host = self.server.server_address[0]