Remove the code nothing in production calls (closes #312)

Service.Warm, the ImageCache interface, both GenerateSignedURL methods,
signature.ParseParams, three functions of internal/magic, the image
processor's lists of supported formats, IsEmpty and Count of the
allowlist, ssrfSafeDialer, CacheStale, ErrCacheMiss, ErrNegativeCache,
ImageResponse.LastModified and VariantStorage.Load had no caller outside
tests. They are gone, with the tests that tested only them.

The handlers tests sign their image URLs with generateSignedURL, a test
helper that builds the URL as the removed generator did. The SSRF dialer
tests call dialSSRFSafe with no operator blocklist, which is all
ssrfSafeDialer did. ParseImageURL, MIMEToImageFormat, ContentStorage.Store
and CleanExpired stay, as pixa calls them.

Model: opus-5-5

internal/imgcache/cache.go

@@ -14,12 +14,6 @@ import (
 	"sneak.berlin/go/pixa/internal/imagerequest"
 )
 
-// Cache errors.
-var (
-	ErrCacheMiss     = errors.New("cache miss")
-	ErrNegativeCache = errors.New("negative cache hit")
-)
-
 // maxPendingAccessTimes is how many access times of variants served from
 // the cache, and how many of cached sources a miss used, may each wait to be
 // written, at about 80 bytes each. An access time handed over while that