feat: add user:file_upload OAuth scope

Align OAuth scopes with upstream Claude Code client which now includes
the user:file_upload scope for file upload support.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/internal/pkg/oauth/oauth.go

@@ -21,12 +21,12 @@ const (
 	// OAuth endpoints
 	AuthorizeURL = "https://claude.ai/oauth/authorize"
 	TokenURL     = "https://platform.claude.com/v1/oauth/token"
-	RedirectURI  = "https://platform.claude.com/oauth/code/callback"
+	RedirectURI  = ""
 
 	// Scopes - Browser URL (includes org:create_api_key for user authorization)
-	ScopeOAuth = "org:create_api_key user:profile user:inference user:sessions:claude_code user:mcp_servers"
+	ScopeOAuth = "org:create_api_key user:profile user:inference user:sessions:claude_code user:mcp_servers user:file_upload"
 	// Scopes - Internal API call (org:create_api_key not supported in API)
-	ScopeAPI = "user:profile user:inference user:sessions:claude_code user:mcp_servers"
+	ScopeAPI = "user:profile user:inference user:sessions:claude_code user:mcp_servers user:file_upload"
 	// Scopes - Setup token (inference only)
 	ScopeInference = "user:inference"