fix(files): bound the HEIF fallback decode input (#6348)

Uploads allow 100MB and prepareImageForVision runs sharp with
limitInputPixels: false, so nothing upstream capped what could reach the
single-threaded WebAssembly decoder. A tenant-controlled file could therefore
spend unbounded CPU and memory on one read.

Cap the transcode input at 20MB — generous headroom over any phone photo,
which runs 1-4MB. Pixel-dimension bombs stay bounded by libheif's own
security limits during parse.

apps/sim/lib/uploads/server/heic.test.ts

@@ -76,6 +76,13 @@ describe('isHeifContainer', () => {
 })
 
 describe('transcodeHeicToJpeg', () => {
+  it('refuses to decode above the input ceiling', async () => {
+    // Uploads allow 100MB; without this bound a tenant could spend an unbounded
+    // WASM decode on a single read.
+    const oversized = Buffer.alloc(20 * 1024 * 1024 + 1)
+    expect(await transcodeHeicToJpeg(oversized)).toBeNull()
+  })
+
   it('returns null for bytes libheif cannot decode', async () => {
     // Also proves the dynamic `heic-convert` import resolves at runtime, which no
     // amount of type-checking establishes for a lazily loaded WebAssembly module.

apps/sim/lib/uploads/server/heic.ts

@@ -24,6 +24,18 @@ const HEIF_BRANDS = new Set([
   'avis',
 ])
 
+/**
+ * Byte ceiling for a fallback decode. Uploads allow 100MB and the vision path runs
+ * sharp with `limitInputPixels: false`, so without this a tenant could push an
+ * arbitrarily large HEIF through a single-threaded WebAssembly decode. 20MB leaves
+ * generous headroom over any phone photo — a 12MP iPhone HEIC is 1-4MB — while
+ * bounding what one read can cost.
+ *
+ * This bounds file size, not pixel count. A small file declaring enormous
+ * dimensions is rejected during parse by libheif's own security limits.
+ */
+const MAX_TRANSCODE_INPUT_BYTES = 20 * 1024 * 1024
+
 /**
  * Whether these bytes are an ISO-BMFF container in the HEIF family.
  *
@@ -58,6 +70,14 @@ export function isHeifContainer(buffer: Buffer): boolean {
  * Returns `null` when the bytes cannot be decoded; never a partial image.
  */
 export async function transcodeHeicToJpeg(buffer: Buffer): Promise<Buffer | null> {
+  if (buffer.length > MAX_TRANSCODE_INPUT_BYTES) {
+    logger.warn('Skipped HEIC transcode above the input ceiling', {
+      bytes: buffer.length,
+      ceiling: MAX_TRANSCODE_INPUT_BYTES,
+    })
+    return null
+  }
+
   try {
     const convert = (await import('heic-convert')).default
     const jpeg = await convert({ buffer, format: 'JPEG' })