Detect a fetched image's format from its bytes once (closes #319)

magic.DetectFormat returns an imagerequest.Format. ValidateMagicBytes
returns the format it checked against the upstream Content-Type. The
cache reads a cached source's format from its file's first bytes, as it
reads a variant's. The proxy hands the format to Process. Process
refuses a format that is not a source format, and input libvips would
load with another loader, such as an SVG passing as AVIF.
Format.MIMEType declares each MIME string once, and the fetcher, the
cache's variant read and the Accept matching use it. magic.MIMEType,
magic.ImageFormat, MIMEToImageFormat, magic's SVG detection,
detectFormat, formatFromString, FormatToMIME and Result.InputFormat are
gone.

Model: opus-5-5

README.md

@@ -191,10 +191,11 @@ allowlisted source hosts for open access.
 name.
 
 Each source image and each transformed image has one row in the database,
-holding its size and when it was last used. Two source URLs that serve the same
-bytes are stored twice, once under each URL. A source image is stored by writing
-its file and then its row; when the row cannot be written, the file is removed
-again.
+holding its size and when it was last used. pixa reads a cached source image's
+format from its file's first bytes. When they match none of the source formats,
+pixa fetches the source again. Two source URLs that serve the same bytes are
+stored twice, once under each URL. A source image is stored by writing its file
+and then its row; when the row cannot be written, the file is removed again.
 
 Source and transformed image files are written under a temporary name, synced to
 disk, and only then renamed into place, so a crash leaves no file under its own