Add security hardening and additional auth tests

Security improvements:
- Add password strength validation (min 8 characters)
- Invalidate all sessions on password change/reset
- Add timing-safe user lookup to prevent enumeration attacks
- Fix SQLAlchemy boolean comparisons (== True -> is_(True))
- Change default admin password to 'changeme123' (meets min length)

New tests (7 additional):
- Inactive user login attempt blocked
- Short password rejected on create/change/reset
- Duplicate username rejected (409)
- Non-owner API key deletion blocked (403)
- Sessions invalidated on password change

backend/app/auth.py

@@ -22,6 +22,26 @@ API_KEY_PREFIX = "orch_"
 # Session duration (24 hours default)
 SESSION_DURATION_HOURS = 24
 
+# Password requirements
+MIN_PASSWORD_LENGTH = 8
+
+
+class PasswordTooShortError(ValueError):
+    """Raised when password doesn't meet minimum length requirement."""
+
+    pass
+
+
+def validate_password_strength(password: str) -> None:
+    """Validate password meets minimum requirements.
+
+    Raises PasswordTooShortError if password is too short.
+    """
+    if not password or len(password) < MIN_PASSWORD_LENGTH:
+        raise PasswordTooShortError(
+            f"Password must be at least {MIN_PASSWORD_LENGTH} characters"
+        )
+
 
 def hash_password(password: str) -> str:
     """Hash a password using bcrypt."""
@@ -93,23 +113,37 @@ class AuthService:
         """Authenticate a user with username and password.
 
         Returns the user if authentication succeeds, None otherwise.
+        Uses constant-time comparison to prevent timing-based user enumeration.
         """
         user = self.get_user_by_username(username)
+        
+        # Always perform password verification to prevent timing attacks
+        # Use a dummy hash if user doesn't exist
+        dummy_hash = "$2b$12$LQv3c1yqBWVHxkd0LHAkCOYz6TtxMQJqhN8/X4.VTtYA1vQ9S9sXa"
+        password_hash = user.password_hash if user and user.password_hash else dummy_hash
+        
+        # Verify password (constant time even if user doesn't exist)
+        password_valid = verify_password(password, password_hash)
+        
+        # Check all conditions
         if not user:
             return None
         if not user.password_hash:
             return None  # OIDC-only user
         if not user.is_active:
             return None
-        if not verify_password(password, user.password_hash):
+        if not password_valid:
             return None
         return user
 
     def change_password(self, user: User, new_password: str) -> None:
-        """Change a user's password."""
+        """Change a user's password and invalidate all existing sessions."""
+        validate_password_strength(new_password)
         user.password_hash = hash_password(new_password)
         user.must_change_password = False
         self.db.commit()
+        # Invalidate all existing sessions for security
+        self.delete_user_sessions(user)
 
     def update_last_login(self, user: User) -> None:
         """Update the user's last login timestamp."""
@@ -120,7 +154,7 @@ class AuthService:
         """List all users."""
         query = self.db.query(User)
         if not include_inactive:
-            query = query.filter(User.is_active == True)
+            query = query.filter(User.is_active.is_(True))
         return query.order_by(User.username).all()
 
     def set_user_active(self, user: User, is_active: bool) -> None:
@@ -134,10 +168,13 @@ class AuthService:
         self.db.commit()
 
     def reset_user_password(self, user: User, new_password: str) -> None:
-        """Reset a user's password (admin action)."""
+        """Reset a user's password (admin action) and invalidate all sessions."""
+        validate_password_strength(new_password)
         user.password_hash = hash_password(new_password)
         user.must_change_password = True
         self.db.commit()
+        # Invalidate all existing sessions for security
+        self.delete_user_sessions(user)
 
     # --- Session Operations ---
 
@@ -159,7 +196,8 @@ class AuthService:
         session = UserSession(
             user_id=user.id,
             token_hash=token_hash,
-            expires_at=datetime.now(timezone.utc) + timedelta(hours=SESSION_DURATION_HOURS),
+            expires_at=datetime.now(timezone.utc)
+            + timedelta(hours=SESSION_DURATION_HOURS),
             user_agent=user_agent,
             ip_address=ip_address,
         )
@@ -328,7 +366,7 @@ def create_default_admin(db: Session) -> Optional[User]:
     auth_service = AuthService(db)
     admin = auth_service.create_user(
         username="admin",
-        password="admin",
+        password="changeme123",
         is_admin=True,
         must_change_password=True,
     )