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/cache_hit_internal_test.go

@@ -49,7 +49,8 @@ func TestVariant_ContentTypeFromFirstBytes(t *testing.T) {
 
 		key := imagerequest.VariantKey("aabbccdd10" + string(format))
 
-		err = cache.PutVariant(t.Context(), key, bytes.NewReader(image))
+		err = cache.PutVariant(t.Context(), key, SourceInfo{CheckedAt: time.Now()},
+			bytes.NewReader(image))
 		if err != nil {
 			t.Fatalf("PutVariant(%s) error = %v", format, err)
 		}