Fix chunk delete deadlock ordering

[oldnicke]

Summary

• pre-delete memory_links for chunks in deterministic key order before deleting chunks
• delete chunk rows through an ordered CTE so concurrent chunk removals acquire locks consistently
• add regression coverage for the ordered delete sequence

Why

Concurrent DELETE FROM chunks ... statements can let PostgreSQL's FK cascade lock overlapping memory_links rows in executor-chosen order. When two writers delete overlapping chunk graphs, that can produce AB/BA row-lock deadlocks. Explicitly deleting the affected links in a stable order removes the unordered cascade as the first lock owner.

Fixes #2560.

Supersedes #2569, which was closed when the fork branch was renamed.

Validation

• uv run --project hindsight-api-slim pytest -q hindsight-api-slim/tests/test_chunk_storage_delete_ordering.py
• uv run --project hindsight-api-slim ruff format --check hindsight-api-slim/hindsight_api/engine/retain/chunk_storage.py hindsight-api-slim/tests/test_chunk_storage_delete_ordering.py
• uv run --project hindsight-api-slim ruff check hindsight-api-slim/hindsight_api/engine/retain/chunk_storage.py hindsight-api-slim/tests/test_chunk_storage_delete_ordering.py