feat: custom recipe cover color/icon + mute auto placeholder palette (v0.51.0)

Two changes to the no-photo cover placeholder shipped last version:

1. Muted the gradient palette (100/40-opacity tints instead of solid -200/
   -950 stops) — the original was too saturated next to real cover photos
   in the same grid, per feedback.
2. New coverIcon/coverColor columns on recipes (nullable text, migration
   0048, additive-only) let the author pin a specific color+icon from the
   recipe editor instead of the automatic per-id pick. getRecipePlaceholder
   now checks these first, falling back to the deterministic hash pick when
   unset — existing recipes are unaffected until edited.

Wired coverIcon/coverColor through every explicit-column recipe select
that feeds a grid card (explore trending/recent, search, feed, for-you) —
the relational-query call sites (recipes page, collections, profile pages)
already return all columns and needed no changes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/messages/en.json

@@ -872,6 +872,9 @@
     "nutritionToggleHelp": "Per serving. Turns off the AI-estimate action on the recipe page — clear these fields (turn this off) to let AI estimate it instead.",
     "calories": "Calories (kcal)",
     "photos": "Photos",
+    "cover": "Cover",
+    "coverHint": "Only used when this recipe has no photo.",
+    "coverReset": "Reset to automatic",
     "ingredients": "Ingredients",
     "amount": "Amount",
     "unit": "Unit",

apps/web/messages/fr.json

@@ -863,6 +863,9 @@
     "nutritionToggleHelp": "Par portion. Désactive l'estimation IA sur la page de la recette — videz ces champs (désactivez ceci) pour laisser l'IA l'estimer à nouveau.",
     "calories": "Calories (kcal)",
     "photos": "Photos",
+    "cover": "Couverture",
+    "coverHint": "Utilisé uniquement si cette recette n'a pas de photo.",
+    "coverReset": "Réinitialiser en automatique",
     "ingredients": "Ingrédients",
     "amount": "Quantité",
     "unit": "Unité",