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/api/src/routes/pipeline.ts

@@ -17,7 +17,7 @@ import { validateImageBuffer } from "../lib/file-validation.js";
 import { sanitizeFilename } from "../lib/filename.js";
 import { createWorkspace } from "../lib/workspace.js";
 import { requireAuth } from "../plugins/auth.js";
-import { getToolConfig } from "./tool-factory.js";
+import { getRegisteredToolIds, getToolConfig } from "./tool-factory.js";
 
 /** Schema for a single pipeline step. */
 const pipelineStepSchema = z.object({
@@ -312,5 +312,15 @@ export async function registerPipelineRoutes(app: FastifyInstance): Promise<void
     },
   );
 
+  /**
+   * GET /api/v1/pipeline/tools
+   *
+   * Returns the IDs of tools that can be used as pipeline steps.
+   * Only tools registered via createToolRoute() support pipeline execution.
+   */
+  app.get("/api/v1/pipeline/tools", async (_request: FastifyRequest, reply: FastifyReply) => {
+    return reply.send({ toolIds: getRegisteredToolIds() });
+  });
+
   app.log.info("Pipeline routes registered");
 }

apps/api/src/routes/tool-factory.ts

@@ -48,6 +48,14 @@ export function getToolConfig(toolId: string): AnyToolRouteConfig | undefined {
   return toolRegistry.get(toolId);
 }
 
+/**
+ * Return the IDs of all tools registered via createToolRoute().
+ * These are the tools that can be used in pipelines and batch processing.
+ */
+export function getRegisteredToolIds(): string[] {
+  return [...toolRegistry.keys()];
+}
+
 /**
  * Factory that registers a POST /api/v1/tools/:toolId route.
  *