recipes: replace placeholder images with OKLAB dominant color backgrounds

Instead of generating/serving 20px placeholder images with blur CSS, extract
a perceptually accurate dominant color (Gaussian-weighted OKLAB average) and
use it as a solid background-color while the full image loads. Removes
placeholder image generation, blur CSS/JS, and placeholder directory references
across upload flows, API routes, service worker, and all card/hero components.
Adds admin bulk tool to backfill colors for existing recipes.

src/models/Recipe.ts

@@ -12,6 +12,7 @@ const RecipeSchema = new mongoose.Schema(
 	    mediapath: {type: String, required: true}, // filename with hash for cache busting: e.g., "maccaroni.a1b2c3d4.webp"
 	    alt: String,
 	    caption: String,
+	    color: String, // dominant color hex e.g. "#a1b2c3", used as loading placeholder
     }],
     description: {type: String, required: true},
     note: {type: String},