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>

docs/ARCHITECTURE.md

@@ -259,6 +259,19 @@ automatic** (D-021).
   unarchived dinner still `pending` with no `ground_dinner` job for its revision
   queued or running is set `failed` with `interrupted`, so no dinner stays
   pending with nothing left to ground it.
+  Photo backfill. `POST /api/v1/settings/photo-backfill` (CSRF-checked)
+  enqueues a `backfill_dinner_photos` job and answers 202 `{id}` without
+  waiting for it; the kind is refused by `POST /api/v1/jobs`. The job
+  grounds, one at a time with `Ground` on its name, every unarchived dinner
+  that is `failed` with no photo and no pasted `recipe_url` — grounded,
+  `manual` and `pending` dinners are never listed, and a pasted link is
+  never replaced by a search. Success sets grounding_status `grounded` as
+  the `ground_dinner` job does (the recipe keeps its source URL and site);
+  failure leaves the dinner `failed` with the new `grounding_error`, for
+  the fix-recipe flow. One dinner's failure never stops the others; a
+  dinner renamed, archived or grounded by hand meanwhile is left as it is.
+  Its result is `{dinners: [{dinner_id, outcome, reason}]}`, outcome
+  `grounded`, `failed` (with its reason) or `skipped`.
   `GET /api/v1/dinners/{id}/recipe-candidates`
   searches for the dinner's name and returns `{candidates: [{candidate_id,
   title, site_name, photo_url, minutes}]}`, at most 3, ranked as `Ground`