feat(29-01): add dominant color extraction via Sharp

extractDominantColor() resizes image to 1x1 pixel for weighted average
color. Integrated into fetchImageFromUrl to return dominantColor.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/server/services/image.service.ts

@@ -1,4 +1,5 @@
 import { randomUUID } from "node:crypto";
+import sharp from "sharp";
 import { uploadImage } from "./storage.service";
 
 const ALLOWED_TYPES = ["image/jpeg", "image/png", "image/webp"];
@@ -8,6 +9,7 @@ const FETCH_TIMEOUT = 10_000; // 10 seconds
 interface FetchImageResult {
 	filename: string;
 	sourceUrl: string;
+	dominantColor: string | null;
 }
 
 export async function fetchImageFromUrl(
@@ -75,5 +77,29 @@ export async function fetchImageFromUrl(
 	// Upload to object storage
 	await uploadImage(Buffer.from(buffer), filename, contentType);
 
-	return { filename, sourceUrl: url };
+	const dominantColor = await extractDominantColor(buffer);
+
+	return { filename, sourceUrl: url, dominantColor };
+}
+
+/**
+ * Extract the dominant color from an image buffer.
+ * Resizes to 1x1 pixel for a perceptually weighted average.
+ * Returns hex string like '#a3b2c1' or null on failure.
+ */
+export async function extractDominantColor(
+	buffer: Buffer | ArrayBuffer,
+): Promise<string | null> {
+	try {
+		const { data } = await sharp(Buffer.from(buffer))
+			.resize(1, 1)
+			.raw()
+			.toBuffer({ resolveWithObject: true });
+		const r = data[0];
+		const g = data[1];
+		const b = data[2];
+		return `#${r.toString(16).padStart(2, "0")}${g.toString(16).padStart(2, "0")}${b.toString(16).padStart(2, "0")}`;
+	} catch {
+		return null;
+	}
 }