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/app/(app)/recipes/[id]/edit/page.tsx

@@ -60,6 +60,8 @@ export default async function EditRecipePage({ params }: Params) {
       isCover: photo.isCover,
       preview: getPublicUrl(photo.storageKey),
     })),
+    coverIcon: recipe.coverIcon,
+    coverColor: recipe.coverColor,
     isBatchCook: recipe.isBatchCook,
     // Only pre-fill the manual-nutrition editor with existing values when
     // they were actually manually entered — AI-estimated data shouldn't