Initial scaffold: FastAPI backend + Vue 3 frontend + Docker setup

Backend: config/db/security/logging core, SQLAlchemy models (Server,
Credential, UpdateJob, UpdateLog, AuditLog, User), services (winrm, ssh,
cau, audit, job_runner), REST API (auth, servers, updates, audit),
Socket.io WebSocket layer.
Frontend: Vue 3 + TS + Pinia + Tailwind, Views (Dashboard, Servers,
Updates, Audit, Login), axios + socket.io-client, nginx prod config.

backend/app/core/exceptions.py

@@ -0,0 +1,66 @@
+"""Custom application exceptions, mapped to HTTP responses in main.py."""
+
+
+class AppError(Exception):
+    """Base class for application errors."""
+
+    status_code: int = 500
+    detail: str = "Internal server error"
+
+    def __init__(self, detail: str | None = None) -> None:
+        super().__init__(detail or self.detail)
+        if detail:
+            self.detail = detail
+
+
+class NotFoundError(AppError):
+    status_code = 404
+    detail = "Resource not found"
+
+
+class ConflictError(AppError):
+    status_code = 409
+    detail = "Resource conflict"
+
+
+class UnauthorizedError(AppError):
+    status_code = 401
+    detail = "Authentication required"
+
+
+class ForbiddenError(AppError):
+    status_code = 403
+    detail = "Permission denied"
+
+
+class InvalidTokenError(UnauthorizedError):
+    detail = "Token is invalid or expired"
+
+
+class CredentialDecryptionError(AppError):
+    status_code = 500
+    detail = "Stored credential cannot be decrypted"
+
+
+class ConnectionTestError(AppError):
+    status_code = 502
+    detail = "Connection test failed"
+
+
+class WinRMError(AppError):
+    status_code = 502
+    detail = "WinRM operation failed"
+
+
+class SSHError(AppError):
+    status_code = 502
+    detail = "SSH operation failed"
+
+
+class CAUError(AppError):
+    status_code = 502
+    detail = "Cluster-Aware Updating operation failed"
+
+
+class JobNotCancellableError(ConflictError):
+    detail = "Job cannot be cancelled in its current state"