fix(retain): enqueue relink victims before delta chunk deletion

[koriyoshi2041]

Problem

Delta retain deletes changed and removed chunks directly. The cascade drops their memory links, but unlike full document replacement, this path does not enqueue surviving units whose outgoing temporal or semantic links pointed at the deleted facts. Graph maintenance runs afterwards, but by then the links needed to discover those victims are gone.

This is the relink-victim residue called out as out of scope in #3384.

Fix

Reuse the outgoing fact IDs already collected for observation invalidation and enqueue relink victims before the chunk/link cascade. The store abstraction keeps this a no-op for stores that own links inline.

The existing real-Postgres endpoint-deletion test now also asserts that the surviving source whose outgoing link targeted the deleted chunk is present in graph_maintenance_queue.

Test

• uv run pytest tests/test_chunk_storage_delete_ordering.py::test_delete_chunks_by_ids_sweeps_links_on_both_endpoints -q -n 0 (1 passed)
• uv run ruff check hindsight_api/engine/retain/chunk_storage.py tests/test_chunk_storage_delete_ordering.py
• uv run ruff format --check hindsight_api/engine/retain/chunk_storage.py tests/test_chunk_storage_delete_ordering.py
• git diff --check

Fixes the relink-victim follow-up noted in #3384.