Refuse SVG at the fetcher and never save an unknown input as JPEG (closes #68)

The fetcher no longer accepts image/svg+xml, so an SVG source gets the
refusal every unaccepted upstream type gets, which both image routes now
answer with 415 where they answered 500. orig of an input the processor
cannot name is ErrUnsupportedInputFormat, also answered 415, where it
was saved as JPEG. README.md drops SVG from the source formats and lists
the 415.

Model: opus-5-5

TODO.md

@@ -30,6 +30,16 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-13 SVG sources are refused, and `orig` is never saved as JPEG in place
+  of the source's own format (closes #68): the upstream fetch no longer accepts
+  `image/svg+xml`, which libvips would render. Both image routes answer 415 for
+  an upstream `Content-Type` that is not one of the source formats, SVG
+  included, where they answered 500. `orig` of a source in a format the image
+  processor cannot name is `ErrUnsupportedInputFormat`, also answered 415, where
+  it was saved as JPEG. `README.md` no longer lists SVG. The SVG detection in
+  `internal/magic` stays: the request path does not call it, and it goes with
+  the unused code of https://git.eeqj.de/sneak/pixa/issues/168. An SVG sent
+  under another image type still fails the first-bytes check and answers 500.
 - 2026-10-12 per-client limit on the image routes (closes #236): the new setting
   `image_requests_per_minute` (`PIXA_IMAGE_REQUESTS_PER_MINUTE`), default 600,
   is how many `GET` and `HEAD` requests one client may send to `/v1/image/` and