⚡ Bolt: Use connection pooling for concurrent Supabase storage uploads

[kourdroid]

💡 What: Implemented __aenter__ and __aexit__ in SupabaseStorage to support acting as an asynchronous context manager, sharing a single httpx.AsyncClient across sequential/concurrent upload requests. Updated the /ingest route to wrap storage upload logic inside an async with block, and updated relevant test mocks to behave correctly as context managers.

🎯 Why: Previously, during PDF splitting in the /ingest route, the system instantiated a new SupabaseStorage client, but the underlying HTTP client (httpx.AsyncClient) was instantiated inside the upload method on every call. This meant that each concurrent upload (even though batched using asyncio.gather) was opening a brand new TCP connection and performing a full TLS handshake, causing a severe N+1 latency bottleneck.

📊 Impact: Reduces storage upload connection overhead from O(N) to O(1) by reusing a single HTTP client connection pool across all uploaded split chunks.

🔬 Measurement: Verify by running pytest tests/test_api_ingest.py and reviewing logs for the /ingest endpoint to ensure uploads still succeed but execute significantly faster under load.

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PR created automatically by Jules for task 3448507274474855146 started by @kourdroid

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