⚡ Optimize marketplace listing endpoint with async/parallel execution

[miethe]

• 💡 What:
  • Refactored list_listings in skillmeat/api/routers/marketplace.py to use asyncio.to_thread for broker_inst.listings calls.
  • Implemented parallel execution of broker queries using asyncio.gather.
  • Added a test fixture in tests/api/test_marketplace_router.py to clear the global cache between tests, resolving test pollution issues.
• 🎯 Why:
  • The MarketplaceBroker.listings method performs blocking synchronous HTTP requests (using requests), which was blocking the main event loop and preventing concurrent handling of other requests.
  • Queries were executed sequentially, meaning the total latency was the sum of all broker latencies.
• 📊 Measured Improvement:
  • Baseline (Blocking): 3.00s for 3 brokers with 1s simulated delay each. No event loop activity (heartbeat stalled).
  • Optimized (Async/Parallel): 1.01s for the same scenario. Event loop remained responsive (heartbeat active).
  • Result: ~3x speedup in this scenario and non-blocking execution.

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PR created automatically by Jules for task 8803127839114561196 started by @miethe

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