fix(pipeline): unblock integration review budget

[ivanarama]

Summary

• allow only a signed, exact integration-review occurrence to use the active P1 MERGE headroom
• persist that temporary priority across budget defer/reclaim, then restore it when the route changes
• preserve explicit task/series priority edits and keep recurring REVIEW successors at their configured priority
• account for shared GitHub headroom across profiles

Safety

• ordinary content REVIEW remains P2
• promotion requires the signed fallback lease and exact replica target reservation
• DB transitions are fenced by task id plus started_at
• invalid restoration fails closed

Tests

• python -m pytest -q (634 passed)
• python -m pytest -q tests/test_github_budget.py tests/test_pipeline_replicas.py (213 passed)
• python -m compileall -q promptpilot
• git diff --check