[GrantFox][High] Add upstream proxy timeouts and circuit breaking

[greatest0fallt1me]

Summary

Add upstream proxy timeouts and circuit breaking

Why this matters

Slow or unhealthy upstreams can exhaust application connections.

Scope

Set finite connect/header/body deadlines, retry only safe methods, and implement open/half-open/closed circuit states.

Acceptance criteria

• Every request has a finite total deadline.
• Unsafe mutations are never automatically retried.
• Circuit recovery requires a successful probe.
• Tests cover slow headers, resets, retry limits, and recovery.

Validation

Add regression coverage for existing behavior, failure modes, authorization boundaries, and compatibility. The implementation must pass the repository CI checks.

Non-goals

• Typo-only, formatting-only, or documentation-only changes.
• Unrelated refactors or dependency upgrades.
• Weakening existing security, authorization, CI, or production safeguards.

Contributor application

Before implementation, comment with relevant experience, a 1–4 bullet approach, and an estimate for opening the first draft PR. Wait for maintainer assignment before coding.

PR requirements

Use a feature branch, include Closes #<issue-number>, check every acceptance criterion, link criteria to code/tests, explain security and failure-mode considerations, and pass CI.

Reward-readiness

This is a substantive GrantFox campaign issue. Merge and CI success do not by themselves guarantee reward eligibility; final reward-readiness is determined by campaign review.