Fix auth bypasses, race condition, stale connections, and unbounded query

- Block disabled accounts at login and unimpersonate (C4, C2)
- Catch IntegrityError on register commit to return 409 instead of 500 (C5)
- Stop _seed_admin from overwriting existing admin password on restart (C8)
- Cap public/stats query at 500 users to bound memory usage (C3)
- Rate-limit PUT /me/password at 5/minute (C6)
- Bump SQLAlchemy to 2.0.50 and aiomysql to 0.3.2; re-enable pool_pre_ping (C7)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/requirements.txt

@@ -1,7 +1,7 @@
 fastapi==0.115.6
 uvicorn[standard]==0.32.1
-sqlalchemy[asyncio]==2.0.36
-aiomysql==0.2.0
+sqlalchemy[asyncio]==2.0.50
+aiomysql==0.3.2
 pydantic-settings==2.7.0
 python-jose[cryptography]==3.3.0
 passlib[bcrypt]==1.7.4