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/api/v1/recipes/route.ts

@@ -51,6 +51,8 @@ const CreateRecipeSchema = z.object({
     key: z.string().min(1).max(500),
     isCover: z.boolean().default(false),
   })).max(20).default([]),
+  coverIcon: z.string().max(50).nullable().optional(),
+  coverColor: z.string().max(50).nullable().optional(),
   isBatchCook: z.boolean().default(false),
   dishes: z.array(z.object({
     name: z.string().min(1).max(100),
@@ -142,6 +144,8 @@ export async function POST(req: NextRequest) {
       aiGenerated: data.aiGenerated ?? false,
       language: data.language,
       sourceUrl: data.sourceUrl,
+      coverIcon: data.coverIcon ?? null,
+      coverColor: data.coverColor ?? null,
       isBatchCook: data.isBatchCook,
       nutritionData: data.nutrition ? { perServing: data.nutrition } : undefined,
       nutritionManual: !!data.nutrition,