Introduces a materialized cache table to store blocked issue IDs,
replacing the expensive recursive CTE computation that was causing
~752ms query times on 10K databases (bd-5qim).
The cache is maintained via invalidation on dependency and status
changes, reducing GetReadyWork from O(n²) recursive traversal to
O(1) cache lookup.
Technical details:
- New blocked_issues_cache table with single issue_id column
- ON DELETE CASCADE ensures automatic cleanup
- Migration populates cache using existing recursive CTE logic
- rebuildBlockedCache() fully rebuilds cache on invalidation
- execer interface allows both *sql.DB and *sql.Tx usage
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Co-Authored-By: Claude <noreply@anthropic.com>
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