Add admin panel, WebSocket support, and API versioning

Introduce an admin portal (React + Nginx), WebSocket routing, and
API versioning middleware with `/api/v1/` prefix deprecation.
Add master API key authentication, new Prometheus metrics for AI
token consumption and active WebSockets, and extend S3 config
with a public endpoint URL. Update test paths and fixtures to
align with the new routing structure.

sdk/imago_client/exceptions.py

@@ -0,0 +1,25 @@
+"""
+Exceptions for the Imago Client SDK.
+"""
+
+class ImagoError(Exception):
+    """Base exception for all Imago SDK errors."""
+
+class AuthError(ImagoError):
+    """Raised when authentication fails (401)."""
+
+class QuotaError(ImagoError):
+    """Raised when quota limits are reached (429 or 403 quota)."""
+
+class NotFoundError(ImagoError):
+    """Raised when a requested resource is not found (404)."""
+
+class APIError(ImagoError):
+    """Raised when the API returns an unexpected error."""
+    def __init__(self, message: str, status_code: int = None, details: dict = None):
+        super().__init__(message)
+        self.status_code = status_code
+        self.details = details or {}
+
+class PipelineError(ImagoError):
+    """Raised when an error occurs in the AI processing pipeline."""