feat: image upload optimization with sharp, thumbnails, and structured error codes (#304)

- Add sharp for server-side image processing (WebP conversion + thumbnails)
- New shared backend utility for image upload, optimization, and cleanup
- Return structured error codes from upload endpoints (IMAGE_TOO_LARGE, INVALID_TYPE, etc.)
- Frontend error code mapping with i18n support (EN + DE)
- MedicationAvatar tries thumbnail first, falls back to full image
- Error display in MedicationsPage, MobileEditModal, and Auth avatar upload

Closes #302

backend/src/utils/image-upload.ts

@@ -0,0 +1,80 @@
+import { existsSync, unlinkSync } from "node:fs";
+import { writeFile } from "node:fs/promises";
+import { extname, resolve } from "node:path";
+import sharp from "sharp";
+
+export const ALLOWED_IMAGE_MIME_TYPES = ["image/jpeg", "image/png", "image/webp", "image/gif"];
+export const MAX_IMAGE_UPLOAD_BYTES = 10 * 1024 * 1024;
+
+export function getThumbFilename(imageFilename: string): string {
+	const ext = extname(imageFilename);
+	const base = ext ? imageFilename.slice(0, -ext.length) : imageFilename;
+	return `${base}-thumb.webp`;
+}
+
+export function removeImageFiles(imagesDir: string, imageFilename: string): void {
+	const fullPath = resolve(imagesDir, imageFilename);
+	if (existsSync(fullPath)) unlinkSync(fullPath);
+
+	const thumbFilename = getThumbFilename(imageFilename);
+	if (thumbFilename !== imageFilename) {
+		const thumbPath = resolve(imagesDir, thumbFilename);
+		if (existsSync(thumbPath)) unlinkSync(thumbPath);
+	}
+}
+
+export async function streamToBuffer(stream: NodeJS.ReadableStream): Promise<Buffer> {
+	const chunks: Buffer[] = [];
+	let totalSize = 0;
+
+	for await (const chunk of stream) {
+		const buffer = Buffer.isBuffer(chunk) ? chunk : Buffer.from(chunk);
+		totalSize += buffer.length;
+		if (totalSize > MAX_IMAGE_UPLOAD_BYTES) {
+			throw new Error("IMAGE_TOO_LARGE");
+		}
+		chunks.push(buffer);
+	}
+
+	return Buffer.concat(chunks);
+}
+
+export async function writeOptimizedImageSet(
+	imagesDir: string,
+	filePrefix: string,
+	uploadBuffer: Buffer,
+	options?: {
+		maxEdgePx?: number;
+		thumbSizePx?: number;
+		fullQuality?: number;
+		thumbQuality?: number;
+	}
+): Promise<{ filename: string; thumbFilename: string }> {
+	const maxEdgePx = options?.maxEdgePx ?? 1600;
+	const thumbSizePx = options?.thumbSizePx ?? 96;
+	const fullQuality = options?.fullQuality ?? 82;
+	const thumbQuality = options?.thumbQuality ?? 76;
+
+	const filename = `${filePrefix}-${Date.now()}.webp`;
+	const thumbFilename = getThumbFilename(filename);
+
+	const filepath = resolve(imagesDir, filename);
+	const thumbFilepath = resolve(imagesDir, thumbFilename);
+
+	const optimizedBuffer = await sharp(uploadBuffer, { failOn: "error" })
+		.rotate()
+		.resize({ width: maxEdgePx, height: maxEdgePx, fit: "inside", withoutEnlargement: true })
+		.webp({ quality: fullQuality })
+		.toBuffer();
+
+	const thumbBuffer = await sharp(uploadBuffer, { failOn: "error" })
+		.rotate()
+		.resize({ width: thumbSizePx, height: thumbSizePx, fit: "cover", position: "attention" })
+		.webp({ quality: thumbQuality })
+		.toBuffer();
+
+	await writeFile(filepath, optimizedBuffer);
+	await writeFile(thumbFilepath, thumbBuffer);
+
+	return { filename, thumbFilename };
+}