fix: coerce non-string metadata values to strings in MemoryFact.parse_metadata

[handnewb]

Summary

Fix consolidation blocker caused by non-string metadata values in MemoryFact.

Problem

When memory_units contain metadata with non-string values (e.g., {"original_id": 348} from observation bookmarks), the consolidation worker fails with:

ValidationError: metadata.original_id — Input should be a valid string [type=string_type]

This happens because MemoryFact.metadata is typed as dict[str, str] but the parse_metadata validator doesn't coerce non-string dict values.

Root Cause

In engine/response_models.py:253, metadata: dict[str, str]. The @field_validator("metadata") method only handles str → dict coercion (for asyncpg JSONB serialization), not int/other → str coercion for individual values.

Fix

Added value coercion in parse_metadata: after JSON parsing (or on raw dict input), iterate all key-value pairs and convert both to strings with {str(k): str(val) for k, val in v.items()}. This safely handles integer, float, bool, and None values.

Impact

• 394 out of 10,741 memory_units affected (3.7%) in a production bank
• Consolidation was completely blocked with 6,321 pending items
• 71 consecutive consolidation failures before workaround applied

Testing

• Existing tests pass (no behavior change for already-string metadata)
• Verified on bank with 10,741 MUs — consolidation resumed immediately after fix + workaround cleanup
• Backward compatible: string values pass through unchanged

Closes #2622

[benfrank241]

verified this fixes #2622 — reproduced the metadata.original_id string_type error on main and confirmed the coercion resolves it (int → "12345", also handles the asyncpg JSONB-string path, strings unchanged). pushed a regression test (tests/test_response_models.py, 4 cases) since there wasn't one.

one follow-up (not for this PR): the same dict[str, str] typing is on the HTTP response models too — RecallResult.metadata and the memory-unit list/detail models in api/http.py (lines ~376 / ~612). those don't run through MemoryFact.parse_metadata, so a numeric metadata value could still trip a ValidationError on the direct HTTP recall/browse path. worth a separate pass to apply the same coercion there. this PR correctly unblocks the consolidation path.