fix: unstrand backend-capacity board-unblock retries

[ProtocolWarden]

Summary

• add a backend-capacity parking rule in board-unblock for blocked goal/spec-author tasks that have a safe automatic repromotion path
• keep those tasks out of a stranded Blocked state until Rule 7/9 can re-promote them after backend reset
• add regression coverage for the new parking path and its no-repromotion-path guard

Verification

• .venv/bin/pytest tests/unit/entrypoints/maintenance/test_board_unblock.py -q --tb=short
• .venv/bin/ruff check src/operations_center/entrypoints/maintenance/board_unblock.py tests/unit/entrypoints/maintenance/test_board_unblock.py
• .venv/bin/pytest tests/unit/er000_phase0_golden/ -q --tb=short

[ProtocolWarden]

Resolved: PR merged

Self-review concerns — auto-fixing (up to 6 attempts; re-queued if still unresolved):

Failed checks: code_quality