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 stores that format in the source's row, so a variant made from a
cached source uses it too. The proxy hands it to Process, which still
refuses 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

internal/db/migrations/001_schema.sql

@@ -3,10 +3,12 @@
 
 -- Source images, one row per source URL
 -- Files stored at: cache/sources/<ab>/<cd>/<source_key>, where source_key
--- is the SHA-256 of the source's host, path and query
+-- is the SHA-256 of the source's host, path and query. format is the
+-- image's format, detected from its first bytes when it was fetched.
 CREATE TABLE IF NOT EXISTS source_content (
     source_key TEXT PRIMARY KEY,
     size_bytes INTEGER NOT NULL,
+    format TEXT NOT NULL,
     last_accessed_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
 );
 CREATE INDEX IF NOT EXISTS idx_source_content_last_accessed