fix(orphan-pr): seed cache from open PR branches + discard by PR number

[nokternol]

Summary

Fixes the orphan PR cache miss bug that recurs on every container restart.

• Track A (root cause): seedDraftEntries — on startup, read all open PR branches, parse + normalize each document, upsert into cache with branch/pr metadata. After restart all in-flight items are immediately actionable again.
• Track B (safety net): POST /api/inflight/:pr/discard — closes an orphan PR by number when no cache entry exists. UI now routes doDiscard to this endpoint when item.id is missing.
• Brain normalization: normalizeDocument (idempotent schema migration fn) + POST /api/admin/normalize endpoint + weekly Sunday 03:00 cron to keep committed docs in canonical schema form. Webhook extended to normalize the just-merged file before rebuilding the index.

Test plan

• bash scripts/check.sh — all 8 checks green (verified locally)
• gh run watch — CI green
• Manually: capture an idea, restart API container, confirm /review shows full action set (not just Discard-only)
• Manually: with a true orphan PR (no cache entry), confirm Discard closes the PR and item disappears from /review on reload

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