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/dinners.go

@@ -112,6 +112,20 @@ func postDinner(logger *slog.Logger, svc *dinners.Service) http.HandlerFunc {
 	}
 }
 
+// postPhotoBackfill starts a photo backfill over every dinner without a
+// photo and answers 202 with its job id, which GET /jobs/{id} reports on,
+// without waiting for the job.
+func postPhotoBackfill(logger *slog.Logger, svc *dinners.Service) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		id, err := svc.StartPhotoBackfill(r.Context())
+		if err != nil {
+			writeDinnerError(w, r, logger, "start photo backfill", err)
+			return
+		}
+		writeJSON(w, r, logger, http.StatusAccepted, jobCreated{ID: id})
+	}
+}
+
 // patchDinner edits a dinner, recording the session's actor as updated_by.
 func patchDinner(logger *slog.Logger, svc *dinners.Service) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {