Add GET /api/v1/dinners/{id} dinner detail

Returns a dinner with its recipe for the recipe detail screen and
Kitchen reading mode: photo URLs at both cached sizes (400/1200) or
null, ingredients and steps in recipe position order, the source page
for a fetched recipe, grounding status and error, and why_picked, the
reason of the suggestion that created the dinner (null when none).
Unknown and archived dinners are 404. Read in one transaction.

Adds the GetDinnerSuggestionReason query (sqlc regenerated). The
dinners test env now serves photos from the grounder's own store and
data directory so photo URLs can be fetched in tests.

Maestro task 099e9952-7f98-4700-8369-241c589a1e9d.

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

internal/dinners/dinners.go

@@ -572,7 +572,12 @@ func checkUnique(ctx context.Context, q *db.Queries, self, name string, recipeUR
 
 // PhotoURL is where the API serves a list-sized copy of photo id.
 func PhotoURL(id string) string {
-	return fmt.Sprintf("/media/photos/%s-%d.jpg", id, photoListWidth)
+	return photoURL(id, photoListWidth)
+}
+
+// photoURL is where the API serves photo id at stored width size.
+func photoURL(id string, size int) string {
+	return fmt.Sprintf("/media/photos/%s-%d.jpg", id, size)
 }
 
 func fromRow(r db.Dinner) Dinner {