fix(ai): show why a recategorization failed

The route replaced every provider error with a bare "AI provider failed",
so a stopped Ollama looked the same as a timeout or a bad answer. Pass on the
reason (the API key is already redacted) and log it.

backend/src/routes/aiCategorization.ts

@@ -39,9 +39,12 @@ function runOut(run: AiCategorizationRun | null) {
   };
 }
 
+// Provider errors already have the API key redacted, so the reason is safe to pass on.
 function aiFailure(error: unknown): never {
   if (error instanceof HttpError) throw error;
-  throw new HttpError(502, "AI provider failed", "ai_failed");
+  const reason = error instanceof Error ? error.message : String(error);
+  console.warn("AI categorization failed:", reason);
+  throw new HttpError(502, `AI provider failed: ${reason}`, "ai_failed");
 }
 
 router.get(

backend/tests/aiCategorization.test.ts

@@ -326,6 +326,7 @@ describe("AI categorization", () => {
     const timeout = await request(app).post(`/api/print/${timedOut.id}/recategorize`).set(auth()).send({});
     expect(timeout.status).toBe(502);
     expect(timeout.body.code).toBe("ai_failed");
+    expect(timeout.body.error).toBe("AI provider failed: The AI provider did not answer within 0 s");
     expect(providerRequests).toBe(3);
 
     await stopProvider();
@@ -344,6 +345,7 @@ describe("AI categorization", () => {
     const notRetried = await request(app).post(`/api/print/${badRequest.id}/recategorize`).set(auth()).send({});
     expect(notRetried.status).toBe(502);
     expect(notRetried.body.code).toBe("ai_failed");
+    expect(notRetried.body.error).toBe("AI provider failed: AI provider returned HTTP 400");
     expect(providerRequests).toBe(1);
     expect(target.id).toBeTruthy();
   });