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/utils/imageProcessing.ts

@@ -56,8 +56,8 @@ export async function extractDominantColor(input: Buffer | string): Promise<stri
 	const { width, height } = info;
 	const cx = (width - 1) / 2;
 	const cy = (height - 1) / 2;
-	const sigmaX = 0.15 * width;
-	const sigmaY = 0.15 * height;
+	const sigmaX = 0.3 * width;
+	const sigmaY = 0.3 * height;
 
 	let wL = 0, wa = 0, wb = 0, wSum = 0;