fix: review fixes for MCP and API key feature

- Remove spurious ALTER TABLE client_targets DROP DEFAULT from migration
  (Prisma schema drift side-effect unrelated to this PR)
- Surface revoke errors in ApiKeysPage UI via deleteApiKey.isError
- Fix UpdateProjectInput.color type to allow null, removing unsafe cast
  in the update_project MCP tool handler

frontend/src/pages/ApiKeysPage.tsx

@@ -49,8 +49,8 @@ export function ApiKeysPage() {
     try {
       await deleteApiKey.mutateAsync(id);
       setRevokeConfirmId(null);
-    } catch (err) {
-      // error is surfaced inline via mutation state
+    } catch (_err) {
+      // error rendered below the table row via deleteApiKey.error
     }
   }
 
@@ -81,6 +81,14 @@ export function ApiKeysPage() {
         </div>
       )}
 
+      {deleteApiKey.isError && (
+        <div className="mb-4 p-3 bg-red-50 border border-red-200 rounded-lg text-red-700 text-sm">
+          {deleteApiKey.error instanceof Error
+            ? deleteApiKey.error.message
+            : "Failed to revoke API key"}
+        </div>
+      )}
+
       {isLoading ? (
         <div className="text-center py-12 text-gray-400 text-sm">Loading...</div>
       ) : !apiKeys || apiKeys.length === 0 ? (