An image is checked before it is decoded

An upload went straight to libvips: whatever loader recognised the bytes ran (an SVG sent as image/png was rasterised),
nothing bounded how many pixels it would decode to (a small PNG could decode to gigabytes, three times over), a GIF
was loaded frame by frame and never resized, and all of it ran inside the request with nothing limiting how many at
once. A GIF was typed gifv but stayed a .gif, which a gifv player can't play; its metadata was kept; colours lost
their ICC profile without being converted; HEIC was advertised but the bundled libvips can't decode it.

Now:
- only libvips' JPEG, PNG, GIF, WebP and HEIF loaders ever run on an upload (every other loader is blocked);
- the header alone says how big an image would decode, refused above Media:MaxPixels (40 MP) or MaxFrames;
- a still image is shrunk on load, turned by its orientation and brought into sRGB (thumbnail), then written without
  metadata, a profile picture the same way;
- an animated GIF becomes a looping silent H.264 mp4 typed gifv, as on Mastodon (PostMedia.Kind keeps it a gifv),
  and a remote GIF is an image;
- processing runs Media:Concurrency at a time, and uploads have their own rate limit per credential;
- HEIC and HEIF are no longer offered.

Tests: only the upload formats load, the header tells the size, an SVG posing as a PNG and an image too large are
refused before decoding, an animated GIF becomes a gifv and a still one an image, HEIC isn't advertised. The media
scenarios against the pasture (GoToSocial, Mastodon, Misskey, Akkoma, Pixelfed, Smithereen, Vernissage, Castopod,
PeerTube) pass: 329 checks.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub.Tests/Http/MastodonInstanceTests.cs

@@ -45,6 +45,8 @@ namespace PrivaPub.Tests.Http
 			Assert.Equal($"wss://{PrivaPubHost.Host}", v1.Body["urls"]!.Text("streaming_api"));
 			Assert.Equal(PrivaPub.Api.Mastodon.Controllers.StatusesController.MaxPins, v2.Body["configuration"]!["accounts"].Number("max_pinned_statuses"));
 			Assert.Contains("image/avif", v2.Body["configuration"]!["media_attachments"]!["supported_mime_types"]!.AsArray().Select(t => t!.GetValue<string>()));
+			// what the bundled libvips can't decode is not offered
+			Assert.DoesNotContain("image/heic", v2.Body["configuration"]!["media_attachments"]!["supported_mime_types"]!.AsArray().Select(t => t!.GetValue<string>()));
 			Assert.True(v2.Body["registrations"].Flag("enabled"));
 			Assert.True((await anonymous.Get("/nodeinfo/2.1")).Ok().Body.Flag("openRegistrations"));
 			Assert.False(v2.Body["configuration"]!["translation"].Flag("enabled"));