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

A cached image was served until eviction, so a change at the same URL
was never picked up.

After 24 hours a request checks the source with upstream, sending its
stored ETag and Last-Modified. A 304 renews it with no download or
conversion. New bytes replace it, and older variants are converted
again. A failed check serves the cached image and puts the URL in the
negative cache. Images not checked for 30 days are evicted.

Variant rows keep the source's ETag and Last-Modified, so a variant
whose source was evicted is checked the same way.

A variant file's modification time is its source's check time when it
was made or renewed, so a hit reads no database row.

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)
 		}