Use heic-convert for HEIC-to-JPEG conversion

Sharp's prebuilt libvips lacks HEIF codec support. Replace with
heic-convert (pure JS decoder) for reliable HEIC conversion on all
platforms. Existing HEIC files on disk will be converted on-the-fly
when served via /api/files.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/lib/heic.ts

@@ -0,0 +1,15 @@
+// @ts-expect-error no type declarations available
+import heicConvert from 'heic-convert'
+
+/**
+ * Convert HEIC/HEIF buffer to JPEG.
+ * Uses heic-convert (pure JS) since sharp's prebuilt libvips lacks HEIF support.
+ */
+export async function convertHeicToJpeg(buffer: Buffer): Promise<Buffer> {
+  const result = await heicConvert({
+    buffer,
+    format: 'JPEG',
+    quality: 0.9,
+  })
+  return Buffer.from(result)
+}