Add LOD generator service for 3-tier image generation

Sharp-based service that produces thumb (256px), medium (1024px),
and full (pass-through) tiers from an uploaded image buffer.
Skips upscaling when the source is smaller than a tier's width.

backend/services/lod-generator.js

@@ -0,0 +1,66 @@
+const sharp = require('sharp');
+
+/**
+ * LOD tier configuration.
+ * Each tier defines the max width and WebP quality for that level of detail.
+ * The 'full' tier is a pass-through (no resize, original format).
+ */
+const LOD_TIERS = {
+  thumb:  { maxWidth: 256,  quality: 70 },
+  medium: { maxWidth: 1024, quality: 80 },
+  full:   { maxWidth: null, quality: null },
+};
+
+/**
+ * Generate 3-tier LOD images from an uploaded buffer.
+ *
+ * @param {Buffer} buffer  - Original image buffer
+ * @param {string} originalExt - Original file extension (e.g. '.png', '.jpg')
+ * @returns {Promise<{
+ *   thumb:  { buffer: Buffer, ext: string, width: number, height: number },
+ *   medium: { buffer: Buffer, ext: string, width: number, height: number },
+ *   full:   { buffer: Buffer, ext: string, width: number, height: number }
+ * }>}
+ */
+async function generateLOD(buffer, originalExt) {
+  const metadata = await sharp(buffer).metadata();
+  const origWidth = metadata.width;
+  const origHeight = metadata.height;
+
+  // Full tier — pass-through, no transformation
+  const full = {
+    buffer,
+    ext: originalExt,
+    width: origWidth,
+    height: origHeight,
+  };
+
+  // Generate a resized WebP tier, or reuse original if image is already smaller
+  async function makeTier(tierKey) {
+    const { maxWidth, quality } = LOD_TIERS[tierKey];
+
+    if (origWidth <= maxWidth) {
+      // Image is smaller than tier width — convert to WebP but don't upscale
+      const result = sharp(buffer).webp({ quality });
+      const outBuf = await result.toBuffer();
+      return { buffer: outBuf, ext: '.webp', width: origWidth, height: origHeight };
+    }
+
+    // Resize down (width-based, preserve aspect ratio)
+    const result = sharp(buffer)
+      .resize({ width: maxWidth, withoutEnlargement: true })
+      .webp({ quality });
+    const outBuf = await result.toBuffer();
+    const outMeta = await sharp(outBuf).metadata();
+    return { buffer: outBuf, ext: '.webp', width: outMeta.width, height: outMeta.height };
+  }
+
+  const [thumb, medium] = await Promise.all([
+    makeTier('thumb'),
+    makeTier('medium'),
+  ]);
+
+  return { thumb, medium, full };
+}
+
+module.exports = { generateLOD, LOD_TIERS };