YachtMemory - Stay Processing Phase 1C.1e-a
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Additive migration for phase-specific attempt/retry metadata.

No production PHP logic is changed. Existing global attempt_count,
last_attempt_at, next_attempt_at and last_error remain deliberately.
No historical processing data is invented.

From /var/www/app.yachtmemory.net:

1) Inspect:
   cat database/migrations/04_stay_processing_phase_metadata.sql

2) Apply ONCE:
   mysql -u admin -p yachtmemory_derived < database/migrations/04_stay_processing_phase_metadata.sql

3) Syntax check:
   php -l shared_dashboard/scripts/verify_stay_processing_phase_metadata.php

4) READ-ONLY verify:
   php shared_dashboard/scripts/verify_stay_processing_phase_metadata.php

Expected current baseline: 36 rows; E=COMPLETE, G=PENDING, C=PENDING,
O=PENDING; all newly added attempt/retry fields zero/NULL.

The old due indexes are intentionally retained. They are cleaned up only after
the PHP processing logic has been migrated and validated.