Refuse SVG sources and never save an unknown input as JPEG (closes #68)

The upstream fetch no longer accepts image/svg+xml, and both image
routes answer 415, not 500, for any upstream Content-Type outside the
source formats. Before decoding, the image processor names the input's
format from govips' DetermineImageType and refuses, with 415, input
whose first bytes libvips reads as anything but the six source formats,
so an SVG passing the first-bytes check for AVIF never reaches librsvg.
orig of such an input is refused instead of being saved as JPEG.
README.md drops SVG from the source formats.

SVG detection stays in internal/magic, unused on the request path, for
#168. An AVIF image sequence
(brand avis) answers 415.

Model: opus-5-5

README.md

@@ -190,9 +190,12 @@ path under `/v1/` answers 200, in maintenance mode too.
   the cache and the upstream fetch; 403 when the upstream host, or a host it
   redirects to, is `localhost`, ends in `.localhost` or `.local`, or has an
   address in a blocked network (see `blocked_networks`); 429 over
-  `image_requests_per_minute` (below); 502 when the upstream answered with an
-  error status, and for 5 minutes after that for the same source URL; 503 when
-  pixa is busy or in maintenance mode; 500 for any other failure.
+  `image_requests_per_minute` (below); 415 when the upstream answers with a
+  `Content-Type` that is not one of the source formats (below), or with an image
+  whose first bytes libvips reads as another format; 502 when the upstream
+  answered with an error status, and for 5 minutes after that for the same
+  source URL; 503 when pixa is busy or in maintenance mode; 500 for any other
+  failure.
 - `GET` or `HEAD` `/v1/e/<token>/<name>` — an image through an encrypted URL
   (see Encrypted URLs). Needs: nothing but the URL. Answers: 200; 304 when
   `If-None-Match` matches the image's `ETag`; 400 for a token that does not
@@ -200,7 +203,7 @@ path under `/v1/` answers 200, in maintenance mode too.
   expired; 504 when the upstream has not sent its response headers within
   `upstream_fetch_timeout`, but 500 when that time runs out while the image
   itself is still arriving; 400 for an `Accept` header that is not valid, and
-  406, 403, 429, 502, 503 and 500, as for `/v1/image/`.
+  406, 403, 415, 429, 502, 503 and 500, as for `/v1/image/`.
 - `GET /robots.txt` — asks every crawler to stay away (`Disallow: /`). Needs:
   nothing. Answers: 200.
 - `GET /.well-known/healthcheck.json` — JSON with `status` (`ok`), `now`,
@@ -260,9 +263,11 @@ once, is refused with 400.
   both optional (values under Signature Specification). Both are part of what is
   cached, so each value of either is a separate cached image.
 
-The source image may be JPEG, PNG, GIF, WebP, AVIF, JPEG XL or SVG. The
-upstream's `Content-Type` must name its format, and the image's first bytes must
-match it.
+The source image may be JPEG, PNG, GIF, WebP, AVIF or JPEG XL. The upstream's
+`Content-Type` must name its format, and the image's first bytes must match it.
+An upstream answering with any other `Content-Type`, SVG included, is refused
+with 415, and so is an image whose first bytes libvips reads as another format,
+such as an SVG whose first bytes pass the check for AVIF.
 
 With the format `auto`, pixa chooses the format for each request from its
 `Accept` header, in this order: