Add photo backfill for failed dinners without a photo

Task 36c40130-a69a-46ed-a047-2cdc5ee2d2e0 (Feature 23, task 1 of 2).

A backfill_dinner_photos job grounds, by name with the existing
recipes.Grounder.Ground, every unarchived dinner whose grounding failed
and that has no photo and no pasted recipe_url. Ground already tries the
ranked candidates best first and keeps the first whose photo caches and
whose ingredients normalize, reusing stored recipes and photos. Success
records the recipe and photo as grounded; a failure leaves the dinner
failed with the new grounding_error for the fix-recipe flow, without
stopping its siblings. Writes are guarded on status, photo and
grounding_revision, so grounded, manual, pending, renamed and archived
dinners are never touched.

POST /api/v1/settings/photo-backfill starts a full pass and answers 202
with the job id; the kind is service-only.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/grounding_test.go

@@ -556,3 +556,43 @@ func TestPostDinnerEnqueueFailureAnswersFailedDinner(t *testing.T) {
 		t.Errorf("listed = %v, want failed", got)
 	}
 }
+
+func TestPostPhotoBackfillGroundsFailedDinners(t *testing.T) {
+	t.Parallel()
+	e := newDinnersEnv(t).withRecipes(t)
+	e.start(t)
+
+	// No model reply is scripted, so grounding on creation fails.
+	rec, created := callDinners(t, e.h, http.MethodPost, dinnersPath, `{"name":"Chicken curry"}`)
+	if rec.Code != http.StatusCreated {
+		t.Fatalf("POST = %d: %s", rec.Code, rec.Body)
+	}
+	id := created["id"].(string)
+	waitJob(t, e.h, created["grounding_job_id"].(string))
+	if d := listedDinner(t, e.h, id); d["grounding_status"] != "failed" || d["photo_url"] != nil {
+		t.Fatalf("dinner = %v, want failed without a photo", d)
+	}
+
+	e.model.QueueGenerate(recipestest.NormalizedReply("stovetop", 40))
+	rec, started := callDinners(t, e.h, http.MethodPost, "/api/v1/settings/photo-backfill", "")
+	jobID, _ := started["id"].(string)
+	if rec.Code != http.StatusAccepted || jobID == "" {
+		t.Fatalf("POST photo-backfill = %d: %s, want 202 with a job id", rec.Code, rec.Body)
+	}
+	job := waitJob(t, e.h, jobID)
+	want := fmt.Sprintf(`map[dinners:[map[dinner_id:%s outcome:grounded reason:<nil>]]]`, id)
+	if job["status"] != "succeeded" || job["kind"] != "backfill_dinner_photos" || fmt.Sprint(job["result"]) != want {
+		t.Fatalf("job = %v, want backfill_dinner_photos succeeded with %s", job, want)
+	}
+	d := listedDinner(t, e.h, id)
+	photo, _ := d["photo_url"].(string)
+	if d["grounding_status"] != "grounded" || d["grounding_error"] != nil || !strings.HasSuffix(photo, "-400.jpg") {
+		t.Errorf("dinner = %v, want grounded with a photo", d)
+	}
+
+	// The backfill is started only by its own endpoint.
+	rec, _ = callDinners(t, e.h, http.MethodPost, "/api/v1/jobs", `{"kind":"backfill_dinner_photos","payload":{}}`)
+	if rec.Code != http.StatusBadRequest || !strings.Contains(rec.Body.String(), "service_only_kind") {
+		t.Errorf("POST /jobs backfill = %d: %s, want 400 service_only_kind", rec.Code, rec.Body)
+	}
+}