phase6: add provider abstraction, mvp adapter, and ingestion sync tasks

apps/providers/exceptions.py

@@ -0,0 +1,18 @@
+class ProviderError(Exception):
+    """Base provider exception."""
+
+
+class ProviderTransientError(ProviderError):
+    """Temporary provider failure that can be retried."""
+
+
+class ProviderRateLimitError(ProviderTransientError):
+    """Raised when provider rate limit is hit."""
+
+    def __init__(self, message: str, retry_after_seconds: int = 30):
+        super().__init__(message)
+        self.retry_after_seconds = retry_after_seconds
+
+
+class ProviderNotFoundError(ProviderError):
+    """Raised when an unknown provider namespace is requested."""