[Bug]: Adding memories to a just-deleted project returns 500: SessionDeletedError is not mapped to an HTTP status #1770

Describe the bug

POST /api/v2/memories to a project that has just been deleted (or whose deletion is racing the add) returns 500.

SessionDeletedError is raised in packages/server/src/memmachine_server/episodic_memory/episodic_memory_manager.py (lines 156 and 266), but server/api_v2/exceptions.py maps only SessionNotFoundError, and add_memories (server/api_v2/router.py) catches nothing, so the error reaches the client as an unhandled 500.

Steps to reproduce

POST /api/v2/projects/delete  {"org_id": "o", "project_id": "p"}   -> 204
POST /api/v2/memories         {"org_id": "o", "project_id": "p", "messages": [...]}   -> 500

Server log: SessionDeletedError: Session 'o/p' has been deleted.

Expected behavior

A mapped client error: 404 (the project no longer exists, as after the purge completes) or 409/503 (it is being deleted).

Environment

• OS: Linux (Ubuntu 24.04), Docker
• MemMachine Version: main at c99bc0e (0.3.9+50.gc99bc0e), image built from source; also seen at c08cf26
• Development language version: Python 3.12 (in the image)
• Backend: event memory, PostgreSQL 18, Qdrant 1.19.1

Additional context

Suggested fix: map SessionDeletedError in exceptions.py alongside SessionNotFoundError.