⚡ Bolt: Use shared httpx.AsyncClient for concurrent uploads

[kourdroid]

💡 What:
Updated SupabaseStorage to accept an optional httpx.AsyncClient in its upload and download methods. In src/api/routes.py, an httpx.AsyncClient is instantiated and shared across all concurrent file uploads in asyncio.gather.

🎯 Why:
Previously, a new httpx.AsyncClient was instantiated for every split file upload during ingestion. This incurred expensive TCP and TLS handshakes for each HTTP request, creating a performance bottleneck when handling documents with many splits. Attempting to use a request-scoped class context manager (__aenter__) could cause race conditions, so passing the client explicitly is safer.

📊 Impact:
Significantly reduces I/O latency and CPU overhead during PDF split processing. For multi-split PDFs, the total ingest time is expected to drop by eliminating N round-trip handshakes to Supabase storage.

🔬 Measurement:
Profile the latency of the /ingest route when uploading a multi-page PDF (e.g., > 10 pages). Observe the reduction in connection establishment overhead in logging or tracing. The behavior has been verified with existing unit tests.

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PR created automatically by Jules for task 12198355291564608865 started by @kourdroid

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