feat: add deleted-user builtin and safe user deletion

- Add deleted-user as a built-in account (no permissions, cannot log in)
  created during init_wizard, protected from deletion like acc-mgr
- On user delete, reassign all foreign key references to deleted-user
  then delete the original user, instead of failing on IntegrityError
- API keys, notifications, and project memberships are deleted outright
  since they're meaningless without the real user

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

app/init_wizard.py

@@ -295,6 +295,39 @@ def init_acc_mgr_user(db: Session) -> models.User | None:
     return user
 
 
+DELETED_USER_USERNAME = "deleted-user"
+
+
+def init_deleted_user(db: Session) -> models.User | None:
+    """Create the built-in deleted-user if not exists.
+
+    This user serves as a foreign key sink: when a real user is deleted,
+    all references are reassigned here instead of cascading deletes.
+    It has no role (no permissions) and cannot log in.
+    """
+    existing = db.query(models.User).filter(
+        models.User.username == DELETED_USER_USERNAME
+    ).first()
+    if existing:
+        logger.info("deleted-user already exists (id=%d), skipping", existing.id)
+        return existing
+
+    user = models.User(
+        username=DELETED_USER_USERNAME,
+        email="deleted-user@harborforge.internal",
+        full_name="Deleted User",
+        hashed_password=None,
+        is_admin=False,
+        is_active=False,
+        role_id=None,
+    )
+    db.add(user)
+    db.commit()
+    db.refresh(user)
+    logger.info("Created deleted-user (id=%d)", user.id)
+    return user
+
+
 def run_init(db: Session) -> None:
     """Main initialization entry point. Reads config from shared volume."""
     config = load_config()
@@ -319,6 +352,9 @@ def run_init(db: Session) -> None:
     # Built-in acc-mgr user (after roles are created)
     init_acc_mgr_user(db)
 
+    # Built-in deleted-user (foreign key sink for deleted accounts)
+    init_deleted_user(db)
+
     # Default project
     project_cfg = config.get("default_project")
     if project_cfg and admin_user: