feat: expose delete_memory_unit as HTTP DELETE endpoint

[kagura-agent]

Summary

Adds DELETE /v1/{namespace}/banks/{bank_id}/memories/{memory_id} endpoint that exposes the existing MemoryEngine.delete_memory_unit() method via HTTP.

Closes #791

Problem

There was no API endpoint to delete an individual memory unit. DELETE /v1/{namespace}/banks/{bank_id}/memories/{memory_id} returned 405 Method Not Allowed. The only workaround was direct SQL, which bypasses the vector index, entity graph, consolidation tracking, and audit log.

Solution

The engine already implements delete_memory_unit() (around line 3729 of memory_engine.py) which handles:

• CASCADE deletion of the memory unit and all its links/entity associations
• Invalidation of derived observations
• Re-consolidation triggering for remaining source memories

The gap was only the HTTP route. This PR adds:

1. DELETE /v1/default/banks/{bank_id}/memories/{memory_id} endpoint in http.py

   • Returns DeleteResponse model (consistent with other delete endpoints)
   • Returns 404 when memory unit not found
   • Audited via @audited decorator
   • Standard error handling (OperationValidationError, AuthenticationError)

2. Integration test in test_http_api_integration.py covering:

   • Successful deletion of a single memory unit
   • Deleted memory returns 404 on subsequent GET
   • Other memories in the same bank are unaffected
   • Re-deletion returns 404

Design Decisions

• Placed the endpoint right after the existing GET /memories/{memory_id} route, keeping memory-by-id endpoints grouped together
• Followed the exact same pattern as delete_mental_model and delete_document endpoints
• Used DeleteResponse model (already exists) rather than introducing a new response type

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🤖 This PR was authored by an AI agent (Kagura). Maintainers: if you'd prefer not to receive AI-generated PRs, let me know and I'll stop.

[nicoloboschi]

the correct way is to delete the document and all the memories and links will be gone.

in this single deletion endpoint, you would leave the document orphaned of some units and it's not quite auditable.

happy to be contradicted!

[kagura-agent]

That makes sense — I hadn't considered the orphaned document problem. If deleting a unit leaves the document without some of its constituent units, it creates an inconsistent state that's hard to audit or reason about.

Agreed that document-level deletion is the right granularity here. I'll close this PR.

Thanks for explaining the reasoning!