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/auth.py

@@ -3,6 +3,7 @@ import logging
 from fastapi import APIRouter, Depends, HTTPException, Request, status
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy import select
+from sqlalchemy.exc import IntegrityError
 
 from app.dependencies import get_db
 from app.limiter import limiter
@@ -27,7 +28,11 @@ async def register(request: Request, body: UserCreate, db: AsyncSession = Depend
         display_name=body.display_name or body.email.split("@")[0],
     )
     db.add(user)
-    await db.commit()
+    try:
+        await db.commit()
+    except IntegrityError:
+        await db.rollback()
+        raise HTTPException(status_code=status.HTTP_409_CONFLICT, detail="Email already registered")
     await db.refresh(user)
 
     logger.info("New user registered: %s", body.email)
@@ -44,5 +49,9 @@ async def login(request: Request, body: LoginRequest, db: AsyncSession = Depends
         logger.warning("Failed login for: %s", body.email)
         raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid credentials")
 
+    if user.is_disabled:
+        logger.warning("Login attempt on disabled account: %s", body.email)
+        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="Account disabled")
+
     logger.info("Login: %s (id=%s)", body.email, user.id)
     return Token(access_token=create_token(user.id))