* fix(migration): backsweep orphaned observation memory units
Delete observation rows whose every source_memory_id points to a
deleted memory unit, left behind before PR #580 fixed the chunk FK
cascade and before delete_document() called
_delete_stale_observations_for_memories.
Closes#572 (data cleanup for pre-existing installs).
* fix(migration): broaden backsweep to cover all fact types and bank-level orphans
- Pass 1: delete any memory_units row (all fact_types) whose bank_id no
longer exists in banks — catches orphans from bank deletions that
predate a FK cascade between the two tables.
- Pass 2: delete observation rows whose every source_memory_id points to
a deleted memory unit, regardless of document_id/chunk_id anchors.
* test(migration): verify backsweep removes orphans and preserves legit rows
Adds a focused migration test that:
- Starts a fresh pg0 instance at revision f6g7h8i9j0k1
- Seeds orphaned rows for both backsweep passes (ghost-bank + all-dead-sources)
- Seeds legitimate rows that must survive
- Applies the backsweep migration to head
- Asserts the expected rows are deleted/preserved
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