feat(evidence): add S3 storage service and file_uploads model

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/core/config.py

@@ -124,6 +124,13 @@ class Settings(BaseSettings):
         """Check if any AI provider is configured."""
         return self.ANTHROPIC_API_KEY is not None or self.GOOGLE_AI_API_KEY is not None
 
+    # Object Storage (Railway S3-compatible)
+    STORAGE_ENDPOINT: str | None = None
+    STORAGE_ACCESS_KEY: str | None = None
+    STORAGE_SECRET_KEY: str | None = None
+    STORAGE_BUCKET_NAME: str = "resolutionflow-uploads"
+    STORAGE_REGION: str = "us-east-1"
+
     # ConnectWise PSA Integration
     # CW_CLIENT_ID is a product-level GUID registered at developer.connectwise.com
     # All MSP customers share this single clientId — it identifies ResolutionFlow as the integration