Split internal/imgcache into the image proxy and the cache (closes #317)

The new package internal/imageproxy holds Proxy, which was imgcache.Service,
with its Config, ImageResponse, CacheStatus and errors. NewProxy makes and
registers the counters of upstream fetches and transforms. internal/imgcache
holds the cache only, with its hit and miss counters, and imgcache.CacheConfig
is imgcache.Config. The cache reads and writes each kind it keeps with one pair
of methods: Variant and PutVariant, Source and PutSource, Failure and
PutFailure. Source replaces LookupSource followed by GetSourceContent.
imgcache.go and module.go are gone. The proxy's tests moved with it. They reach
the cache through its database, its state directory and its metrics registry,
and run its reconciliation pass with a second cache, as pixad does at startup.

Model: opus-5-5

internal/handlers/server_busy_internal_test.go

@@ -11,8 +11,8 @@ import (
 
 	"sneak.berlin/go/pixa/internal/httpfetcher"
 	"sneak.berlin/go/pixa/internal/imageprocessor"
+	"sneak.berlin/go/pixa/internal/imageproxy"
 	"sneak.berlin/go/pixa/internal/imagerequest"
-	"sneak.berlin/go/pixa/internal/imgcache"
 )
 
 // TestServerBusyAnswers503 checks that respondImageError, which answers the
@@ -58,7 +58,7 @@ func TestProcessingTimedOutAnswers503(t *testing.T) {
 
 	h := &Handlers{log: slog.New(slog.DiscardHandler)}
 	err := fmt.Errorf("%w: failed to encode: %w",
-		imgcache.ErrProcessingTimedOut, context.DeadlineExceeded)
+		imageproxy.ErrProcessingTimedOut, context.DeadlineExceeded)
 
 	rec := httptest.NewRecorder()
 	h.respondImageError(rec, &imagerequest.Request{}, err)