feat: wire in rate limiter with HTTPException handler and env var disable

[shivv23]

Description:
Wire rate limiter into main.py with HTTPException handler
Add rate_limit_enabled config option to disable via env var

Changes:
app/config.py: Add rate_limit_enabled: bool = True
app/main.py: Conditionally add rate limit middleware, add HTTPException handler for 429 responses
Checklist:

✅ Rate limit enforced on training endpoints
✅ 429 returns JSON: {"success": false, "message": "...", "data": null}
✅ Can disable via RATE_LIMIT_ENABLED=false

[ivantha]

The PR imports RateLimitMiddleware and get_training_limiter from app.rate_limiter, but the tensormap-backend/app/rate_limiter.py module is not included in the diff — it appears to be missing from the branch entirely. The application will fail to start with an ImportError. Please add the rate_limiter.py implementation (and any new dependencies in pyproject.toml/uv.lock) to the branch, then this can be re-reviewed.

[shivv23]

You're right @ivantha - the rate_limiter.py implementation is missing from this PR. That's because the implementation was added in PR #288 (which is now mergeable). This PR #277 just wires the rate limiter into main.py, but it depends on #288 being merged first.I'll close this PR and re-create it after #288 is merged so it has the full implementation.