Why this change is needed:
Previously, PostgreSQL and Qdrant had inconsistent migration behavior:
- PostgreSQL kept legacy tables after migration, requiring manual cleanup
- Qdrant auto-deleted legacy collections after migration
This inconsistency caused confusion for users and required different
documentation for each backend.
How it solves the problem:
Unified both backends to follow the same smart cleanup strategy:
- Case 1 (both exist): Auto-delete if legacy is empty, warn if has data
- Case 4 (migration): Auto-delete legacy after successful verification
This provides a fully automated migration experience without manual intervention.
Impact:
- Eliminates need for users to manually delete legacy tables/collections
- Reduces storage waste from duplicate data
- Provides consistent behavior across PostgreSQL and Qdrant
- Simplifies documentation and user experience
Testing:
- All 16 unit tests pass (8 PostgreSQL + 8 Qdrant)
- Added 4 new tests for Case 1 scenarios (empty vs non-empty legacy)
- Updated E2E tests to verify auto-deletion behavior
- All lint checks pass (ruff-format, ruff, trailing-whitespace)
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