feat(storage): add sqlite-vec acceleration for vector search

- Add sqlite-vec virtual table for fast vector search (10s → ~4ms for 10k vectors)
- Add vec_chunks virtual table with automatic migration
- Add MEMOS_USE_VEC_INDEX env var to control indexed search (default: true)
- Automatic fallback to brute-force search if sqlite-vec fails
- Backward compatible: works without sqlite-vec installed
- Add comprehensive tests for vector search functionality

Performance improvement:
- Brute-force: ~10 seconds for 10k vectors
- sqlite-vec indexed: ~4 milliseconds (2500x speedup)

Author: Zuliang Zhao

apps/memos-local-openclaw/package.json

@@ -54,6 +54,7 @@
     "posthog-node": "^5.28.0",
     "puppeteer": "^24.38.0",
     "semver": "^7.7.4",
+    "sqlite-vec": "^0.1.7",
     "uuid": "^10.0.0"
   },
   "devDependencies": {