fix: pipeline only shows compatible tools and displays errors

The pipeline tool picker was showing all tools, but only tools
registered via createToolRoute() support pipeline execution. Tools
with custom routes (remove-background, upscale, ocr, etc.) would
silently fail with "Tool not found" and the empty catch block hid
the error from users.

Add GET /api/v1/pipeline/tools endpoint that returns the IDs of
pipeline-compatible tools. The frontend fetches this list and filters
the tool picker accordingly. Also surface pipeline execution errors
in the UI instead of swallowing them.

apps/web/src/components/tools/pipeline-builder.tsx

@@ -42,6 +42,7 @@ interface PipelineBuilderProps {
     processedSize: number;
     stepsCompleted: number;
   } | null;
+  executionError?: string | null;
 }
 
 export function PipelineBuilder({
@@ -52,6 +53,7 @@ export function PipelineBuilder({
   saving = false,
   executing = false,
   executionResult = null,
+  executionError = null,
 }: PipelineBuilderProps) {
   const [showToolPicker, setShowToolPicker] = useState(false);
   const [expandedStep, setExpandedStep] = useState<string | null>(null);
@@ -61,6 +63,7 @@ export function PipelineBuilder({
   const [file, setFile] = useState<File | null>(null);
   const [disabledTools, setDisabledTools] = useState<string[]>([]);
   const [experimentalEnabled, setExperimentalEnabled] = useState(false);
+  const [pipelineToolIds, setPipelineToolIds] = useState<string[] | null>(null);
 
   useEffect(() => {
     apiGet<{ settings: Record<string, string> }>("/v1/settings")
@@ -71,15 +74,22 @@ export function PipelineBuilder({
         setExperimentalEnabled(data.settings.enableExperimentalTools === "true");
       })
       .catch(() => {});
+
+    // Fetch which tools actually support pipeline execution
+    apiGet<{ toolIds: string[] }>("/v1/pipeline/tools")
+      .then((data) => setPipelineToolIds(data.toolIds))
+      .catch(() => {});
   }, []);
 
   const PIPELINE_TOOLS = useMemo(() => {
     return PIPELINE_TOOLS_BASE.filter((t) => {
       if (disabledTools.includes(t.id)) return false;
       if (t.experimental && !experimentalEnabled) return false;
+      // Only show tools that are registered in the pipeline-compatible tool registry
+      if (pipelineToolIds && !pipelineToolIds.includes(t.id)) return false;
       return true;
     });
-  }, [disabledTools, experimentalEnabled]);
+  }, [disabledTools, experimentalEnabled, pipelineToolIds]);
 
   const addStep = useCallback(
     (toolId: string) => {
@@ -326,6 +336,16 @@ export function PipelineBuilder({
         </button>
       )}
 
+      {/* Execution error */}
+      {executionError && (
+        <div className="rounded-lg border border-red-200 dark:border-red-800 bg-red-50 dark:bg-red-900/20 p-4">
+          <div className="flex items-center gap-2 text-red-700 dark:text-red-400">
+            <icons.AlertCircle className="h-5 w-5 shrink-0" />
+            <span className="text-sm font-medium">{executionError}</span>
+          </div>
+        </div>
+      )}
+
       {/* Execution result */}
       {executionResult && (
         <div className="rounded-lg border border-green-200 dark:border-green-800 bg-green-50 dark:bg-green-900/20 p-4 space-y-2">