Improve error handling: stop swallowing errors and propagate proper HTTP status codes

[devin-ai-integration]

Summary

Fixes several places in main.py where errors were silently swallowed or surfaced as unhelpful 500s.

Relay tasks no longer swallow errors. tcp_to_ws previously had a blanket except Exception: pass, and unexpected exceptions in ws_to_tcp escaped into a task that was never awaited (dropped + "Task exception was never retrieved" warnings). Both now distinguish expected teardown from real failures:

except asyncio.CancelledError: raise          # keep cancellation working
except WebSocketDisconnect: pass              # normal client close
except (ConnectionError, RuntimeError): log.debug(...)   # socket torn down mid-relay
except Exception as exc: record_error(exc, ...); log.error(...)

websocket_tunnel surfaces task failures. After asyncio.wait(..., FIRST_COMPLETED) it now gathers the cancelled pending tasks (clean teardown) and re-raises any exception from the finished task instead of dropping it. The outer handler routes through the new record_error(...) helper (increments stats["total_errors"] + appends to error_logs).

Malformed request bodies return 400, not 500. Added parse_json_body() (rejects non-JSON / non-object bodies) and _to_float() (rejects non-numeric limit_value), wired into api_login, api_change_password, create_link, and toggle_link. Previously await request.json() / float(...) on bad input raised unhandled → 500.

http_proxy differentiates failures. Guards http_client is None (503), maps httpx.TimeoutException → 504, httpx.HTTPError → 502, and logs/records all via record_error.

Verification

Ran the app locally and exercised the paths: invalid-JSON and array bodies → 400, wrong password → 401, valid login → 200, non-numeric limit_value → 400, valid link creation → 200. python -m py_compile passes; the only pyflakes warnings are pre-existing unused imports untouched by this change.

Link to Devin session: https://app.devin.ai/sessions/aecaede8a13e4e1e990020243c687dc7
Requested by: @hmilan1366

[devin-ai-integration]

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