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/app/routers/public.py

@@ -16,7 +16,7 @@ router = APIRouter(prefix="/api/public", tags=["public"])
 @limiter.limit("30/minute")
 async def public_stats(request: Request, db: AsyncSession = Depends(get_db)):
     users_result = await db.execute(
-        select(User).options(selectinload(User.entries))
+        select(User).options(selectinload(User.entries)).limit(500)
     )
     users = users_result.scalars().all()