Phase 3: Fix medium-priority issues (M1-M4, M6)

M1: Password complexity validation (min 8 chars, uppercase, lowercase, digit)
M2: Remove is_system_admin from login response (prevent role leaking)
M3: Permission cache invalidates on DB error instead of using stale data
M4: .env.docker.example already fixed in B9 (SECRET_KEY, FRONTEND_URL, SMTP)
M6: Frontend test setup auto-wraps with QueryClientProvider (fixes ~29 test failures)

Remaining: M5 (frontend component integration — WelcomeDialog, SavedFilterBar, etc.)

app/routes/auth.py

@@ -75,7 +75,6 @@ async def login(
             "email": user.email,
             "name": user.name,
             "role": role,
-            "is_system_admin": user.is_system_admin,
             "tenant_id": str(tenant.id),
             "tenant_name": tenant.name,
             "csrf_token": csrf_token,