FileSize: Add file size limit to audio upload endpoints (`/audio`, `/ws`)

[ashrafee-dev]

Currently, /audio and /ws accept files of any size with no upper bound. file.read() and websocket.receive_bytes() load the entire payload into memory before any validation happens, and filetype.guess() only checks MIME type — not size.

Risks

• A large upload (or many concurrent large uploads) can exhaust server RAM, especially since files are also written to /dev/shm (tmpfs, itself RAM-backed) — a big-enough file could fill available memory twice over (once in the read buffer, once on tmpfs).
• Long transcription time for oversized audio ties up a worker/event-loop slot longer than expected, worsening the concurrency bottleneck already discussed for CPU-only deployment.
• No client feedback today — an oversized file just silently gets processed (or crashes) instead of a clear rejection.

Proposed Fix

Add a MAX_FILE_SIZE constant (e.g. 25 MB — reasonable upper bound for voice notes) and reject anything larger before writing to disk/tmpfs, returning a clear error response.

MAX_FILE_SIZE = 25 * 1024 * 1024  # 25 MB

For /audio:

byte = await file.read()
if len(byte) > MAX_FILE_SIZE:
    return {"error": "File too large. Max size is 25MB."}

For /ws:

byte = await websocket.receive_bytes()
if len(byte) > MAX_FILE_SIZE:
    await websocket.send_json({"error": "File too large. Max size is 25MB."})
    continue

Note: this only rejects after the full payload is already read into memory — for stronger protection (rejecting before the full body is buffered), FastAPI/Starlette also support limiting via Content-Length header pre-check on /audio, or a reverse-proxy-level limit (e.g. nginx client_max_body_size) in front of both endpoints. Worth deciding whether app-level or proxy-level enforcement (or both) fits your deployment.

Acceptance Criteria

• Requests/messages exceeding size limit are rejected with a clear error, not silently processed or crashed on
• Limit is configurable (env var or constant, not hardcoded magic number scattered across files)
• Applies to both /audio and /ws