refactor: extract duplicated code patterns into shared utilities

[devin-ai-integration]

Summary

main.py had several code patterns copy-pasted across the WebSocket relay, HTTP proxy, and link-management endpoints. This refactor extracts them into small shared helpers, leaving behavior unchanged (verified with a functional smoke test over login / link CRUD / stats / health). Net effect: the same logic now lives in one place, so accounting/serialization changes only need to be made once.

Helpers introduced (all in the Helpers section):

• format_uuid_hex(h) — canonical 8-4-4-4-12 formatting, previously duplicated as an inline f-string in both generate_uuid() and format_link_uuid().
• record_traffic(size, *, conn_id=None, link_uid=None, count_request=True) — the 4–5 line byte-accounting block (total_bytes / total_requests / connections[id]["bytes"] / hourly_traffic / add_usage) was repeated 4× in ws_to_tcp, tcp_to_ws, websocket_tunnel, and http_proxy, each a slightly different subset. Flags capture the differences:

  await record_traffic(size, conn_id=conn_id, link_uid=uuid)                    # ws upstream / first chunk
  await record_traffic(size, conn_id=conn_id, link_uid=uuid, count_request=False)  # tcp downstream
  await record_traffic(len(resp.content))                                      # http proxy (no conn/link)

• record_error(exc, **extra) — replaces the stats["total_errors"] += 1; error_logs.append({"error": ..., "time": ...}) pair duplicated in websocket_tunnel and http_proxy; **extra carries the proxy's url=.
• enforce_quota(websocket, link_uid, size) — the "check quota, else ws.close(1008) and break" block duplicated in both relay directions, now if not await enforce_quota(...): break.
• limit_bytes_from_body(body) — the limit_value/limit_unit → bytes parsing duplicated in create_link and toggle_link.
• new_link_record(label, limit_bytes) / serialize_link(uid, data, host) — the link dict literal (built in ensure_default_link + create_link) and the outward JSON shape (built in create_link + list_links) are now single-sourced.
• filter_headers(headers, *, drop_host=False) — the hop-by-hop header-stripping comprehension used twice in http_proxy.

No new dependencies; no endpoint signatures or response shapes changed.

Link to Devin session: https://app.devin.ai/sessions/70a0c90760234f1280795234854adb87
Requested by: @hmilan1366

[devin-ai-integration]

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