Use http server timeout knob in local_backend instead of hard-coded value

[bastikohn]

Main motivation: Allow self-hosted instances to configure the maximum duration for long-running HTTP requests (notably file uploads).

This PR makes the local backend’s HTTP request timeout configurable via HTTP_SERVER_TIMEOUT_SECONDS by standardizing on the shared HTTP_SERVER_TIMEOUT_DURATION knob.

Changes

• Plumb HTTP_SERVER_TIMEOUT_DURATION into both HTTP servers started by local-backend:
  • Main backend listener (API + storage upload endpoints): main.rs
  • Dev site proxy listener (HTTP Actions proxy): proxy.rs
• Adjust the knob default from 300s → 125s in knobs.rs so the knob matches the prior local-backend hardcoded behavior (i.e. no default behavior change for local backend unless the env var is set).
• Expose HTTP_SERVER_TIMEOUT_SECONDS in self-hosted Docker Compose so operators can override it.

Behavior

This timeout is enforced by tower_http::timeout::TimeoutLayer, so it bounds end-to-end request processing time (including time spent receiving an upload body). Requests exceeding the duration will return "408 Request Timeout".
Increasing HTTP_SERVER_TIMEOUT_SECONDS increases the maximum allowed duration for uploads to /api/storage/upload and any long-running HTTP Action requests going through the local backend/proxy.

Testing

Manual verification: start the backend with a low/high HTTP_SERVER_TIMEOUT_SECONDS and confirm long uploads fail/succeed accordingly. (I have verified this using a local build of the backend container.)

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