Fix: use httpx.AsyncClient in async FastAPI handlers to prevent event loop blocking

[selena731]

Replace httpx.Client with httpx.AsyncClient in the listed async FastAPI handlers to avoid blocking the uvicorn event loop. Update code and add tests validating endpoints remain responsive under long-running operations.

Affected files/handlers (from parent issue #219):

• server/routes.py: POST /v1/models/pull (server/routes.py:487)
• server/routes.py: DELETE /v1/models/{name} (server/routes.py:518)
• server/api_routes.py: POST /v1/speech/transcribe (server/api_routes.py:685)
• server/agent_manager_routes.py: SendBlue verify/register/test (lines ~1775,1839,1890)

Implementation notes:

• Use async with httpx.AsyncClient(...) and await client.post/get
• For CPU-bound tasks (e.g., Whisper), offload with asyncio.to_thread
• Add unit/integration tests that mock remote endpoints and simulate long-duration calls

[SoulSniper-V2]

take