fix(producer): classify JSON error bodies as non-media sources too

A source that answers with a JSON error body still reached ffprobe and
produced `moov atom not found`. Replicate returns
`{"detail": "requested file not found"}` for a dead asset, and a gateway
in front of it can relay that body with a success status.

The sniff now treats `<`, `{`, or `[` as the opening byte of a text
document. No supported container starts with any of them, so this is the
same trade as before: three bytes instead of an allowlist that grows one
entry per payload shape observed in production.

Renamed accordingly, since the class now covers JSON as well as markup:
MARKUP_NOT_MEDIA -> NOT_MEDIA_PAYLOAD, MarkupNotMediaError ->
NotMediaPayloadError, markupPayload.ts -> notMediaPayload.ts. Registry
entries in the Lambda name map, the CDK and SAM plan lists, the Cloud Run
set, and SAFE_RENDER_ERROR_CODES move with it.

Also documents the reachability boundary on the error class: only a 2xx
response gets here. `downloadToTemp` rejects 404/410 as `http_not_found`
before writing a byte, and every ffprobe input is local because
videoFrameExtractor downloads http srcs first. So the shapes this
classifies are soft-404 and interstitial HTML, S3/CloudFront error
documents, and JSON API error bodies -- each served with a success
status. A genuine 404 surfaces as a download failure, not as this error.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

packages/engine/src/index.ts

@@ -274,12 +274,12 @@ export {
 } from "./utils/ffprobe.js";
 
 export {
-  MARKUP_NOT_MEDIA,
-  MarkupNotMediaError,
-  assertNotMarkupPayload,
+  NOT_MEDIA_PAYLOAD,
+  NotMediaPayloadError,
+  assertMediaPayload,
   fingerprintElementId,
-  isMarkupPayload,
-} from "./utils/markupPayload.js";
+  isNotMediaPayload,
+} from "./utils/notMediaPayload.js";
 
 export { assertPublicHttpsUrl, downloadToTemp, isHttpUrl } from "./utils/urlDownloader.js";
 export {