⚡ Optimize GitHub scan to run in background thread

[miethe]

Offloaded the blocking scanner.scan_repository call to a separate thread using asyncio.to_thread to prevent freezing the main event loop during repository scans.

💡 What

Modified skillmeat/api/routers/marketplace_sources.py to wrap the synchronous scanner.scan_repository call in asyncio.to_thread.

🎯 Why

Scanning a GitHub repository involves multiple synchronous API calls (e.g., fetching tree, resolving ref) which can take seconds. Running this on the main event loop freezes the server, making it unresponsive to other requests (like health checks or other API calls).

📊 Measured Improvement

Using a reproduction script that mocks a 1-second blocking scan:

• Baseline: The event loop was blocked for ~1.1s (Max event loop gap: 1.1170s).
• Optimized: The event loop continued running with minimal delay (Max event loop gap: 0.1157s).
• Improvement: Reduced blocking time by >90%, restoring responsiveness.

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PR created automatically by Jules for task 8730940876819846466 started by @miethe

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