fix(memory): make local long-term memory work once models are configured

[Luhaozhu]

Summary

• Stale mem0 singleton: saving the memory/embedding model config only invalidated the resolve cache; an instance initialized against the placeholder key kept failing 401 forever. Config routes now reset the memory runtime, and credential errors self-heal by rebuilding the singleton (excluded from the Milvus circuit breaker).
• CE audit stub signature: the stub rejected the real implementation's positional call style, raising after the DB commit and mis-reporting successful L1 writes as failures (no write card in the UI). Now accepts any call shape, with a contract regression test.
• Vector dimension mismatch: collections were created at the configured dimension (default 1024) while embedders return their native width (e.g. 4096). Init now probes the real dimension via one /embeddings call; an existing empty mismatched collection is dropped and rebuilt, a non-empty one (embedding-model switch) is fully re-embedded with the new model before the swap — old data survives if the new model is unreachable.
• Missing local tables: the CE models facade didn't import the evolution models or MemoryRefShadow, so their tables were never created in the local SQLite bootstrap.
• Bumps desktop version to 0.2.16.

Testing

• pytest src/backend/tests/memory — 100 passed (includes new CE overlay contract tests).