Tighten needsFit to exact canvas size, guard against empty renders

needsFit only compared aspect ratio, so a same-aspect-but-larger still
(e.g. a 4032x2268 phone photo against a 1344x768 canvas) would skip
fitting and hand MiniMaxH3AddGuide a still that does not match the
job's own latent size. Require an exact width+height match instead;
anything else (including unreadable dimensions) still fits.

Also guard the video save path: an empty Comfy download, an empty
stitched assembly, or a stitched result with no readable duration now
fails the job with a clear error instead of silently saving/attaching
a broken zero-length clip.

Co-authored-by: Cursor <cursoragent@cursor.com>

server/utils/studio2/media.ts

@@ -12,12 +12,12 @@ export async function fitStill(data: Buffer, width: number, height: number) {
     return readFileSync(output)
   } finally { rmSync(dir,{recursive:true,force:true}) }
 }
-/** Only scale/pad when the still's aspect truly differs from the job canvas.
- * A matching frame (typical extend handoff) must reach Comfy byte-identical. */
+/** Only skip the scale+pad when the still already matches the job canvas pixel-for-pixel.
+ * A same-aspect-but-larger still (e.g. a 4032x2268 phone photo against a 1344x768 canvas)
+ * still needs fitting: MiniMaxH3AddGuide expects an image sized to the job's own latent,
+ * not merely the same aspect ratio. Unreadable dimensions always fit (safe default). */
 export function needsFit(data: Buffer, width: number, height: number) {
   const size = imageDimensions(data)
   if (!size?.width || !size?.height) return true
-  if (size.width === width && size.height === height) return false
-  const target = width / height, actual = size.width / size.height
-  return Math.abs(target - actual) > 0.01
+  return !(size.width === width && size.height === height)
 }