Check cached sources with upstream after the cache TTL (closes #69, closes #83)

A cached source and its variants were served until eviction, so a
changed image at the same URL was never picked up.

Once a source was last checked 24 hours ago, the next request for one
of its images asks upstream with If-None-Match and If-Modified-Since,
from the ETag and Last-Modified it stored. A 304 renews the source and
its variant without a download or a conversion. New bytes replace the
source, and each variant made before them is converted again. A failed
check serves the cached image and puts the URL in the negative cache.
Sources and variants not checked successfully for 30 days are evicted.

A variant file's modification time is its source's check time, so a
hit still makes no database call.

Existing tests change only in call signatures. The test that asserted
two-day-old entries are served without asking upstream is replaced by
the new ones.

Model: opus-5-5

internal/imgcache/source_content_internal_test.go

@@ -128,7 +128,7 @@ func TestPutSource_FailedRowWriteLeavesNoFile(t *testing.T) {
 		Source: imagerequest.Source{Host: testHostCDN, Path: testPathCat},
 	}
 	key, err := cache.PutSource(t.Context(), req.Source,
-		bytes.NewReader([]byte("source bytes")))
+		bytes.NewReader([]byte("source bytes")), SourceInfo{})
 	t.Logf("PutSource() = %q, error = %v", key, err)
 
 	if err == nil {
@@ -170,7 +170,7 @@ func TestSource_FormatFromFirstBytes(t *testing.T) {
 			t.Fatalf("reading the %s image: %v", format, err)
 		}
 
-		_, err = cache.PutSource(t.Context(), src, bytes.NewReader(image))
+		_, err = cache.PutSource(t.Context(), src, bytes.NewReader(image), SourceInfo{})
 		if err != nil {
 			t.Fatalf("PutSource(%s) error = %v", format, err)
 		}