fix: detect and re-run migration 029 when forms tables are missing

[lane711]

Summary

• Fixes Migration ID 029 reused across versions causes cascading failures on upgrade #762: Migration ID 029 was reassigned from "Ai Search Plugin" to "Add Forms System" between releases. Users upgrading from older versions had 029 marked as applied for the wrong migration, causing forms tables to never be created and cascading failures for migrations 030 and 033.
• Adds bidirectional auto-detection for migration 029 in autoDetectAppliedMigrations(): if 029 is recorded but forms/form_submissions/form_files tables don't exist, the stale record is removed so the forms migration re-runs on next bootstrap.
• Improves admin API error messaging to include per-migration failure details instead of a generic "Failed to run migrations".

Changes

File	Description
packages/core/src/services/migrations.ts	Add 029 auto-detection (same pattern as existing 011 check); add errors array to runPendingMigrations() return type
packages/core/src/routes/admin-api.ts	Pass through per-migration errors in POST /migrations/run response
packages/core/src/services/migrations.test.ts	7 unit tests covering all detection scenarios
docs/ai/plans/fix-762-migration-029-reuse-plan.md	Implementation plan

Testing

• Unit tests pass locally (1443 tests, 0 failures)
• TypeScript compiles with no errors
• New tests cover all 4 detection scenarios for migration 029

Test plan

• Verify on a fresh D1 database that all migrations apply correctly
• Verify on an upgraded database (with old 029 = Ai Search) that forms migration re-runs
• Verify admin UI shows specific migration errors instead of generic message

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