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/httpfetcher/httpfetcher_internal_test.go

@@ -210,7 +210,7 @@ func TestMockFetcher_FetchesFile(t *testing.T) {
 	m := NewMock(mockFS)
 
 	result, err := m.Fetch(context.Background(),
-		imagerequest.Source{Host: testHost, Path: "/images/photo.jpg"})
+		imagerequest.Source{Host: testHost, Path: "/images/photo.jpg"}, "", "")
 	if err != nil {
 		t.Fatalf("Fetch() error = %v", err)
 	}
@@ -242,7 +242,7 @@ func TestMockFetcher_MissingFileReturnsUpstreamError(t *testing.T) {
 	m := NewMock(mockFS)
 
 	_, err := m.Fetch(context.Background(),
-		imagerequest.Source{Host: testHost, Path: "/missing.jpg"})
+		imagerequest.Source{Host: testHost, Path: "/missing.jpg"}, "", "")
 	if !errors.Is(err, ErrUpstreamError) {
 		t.Errorf("Fetch() error = %v, want ErrUpstreamError", err)
 	}
@@ -259,7 +259,7 @@ func TestMockFetcher_RespectsContextCancellation(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()
 
-	_, err := m.Fetch(ctx, imagerequest.Source{Host: testHost, Path: "/photo.jpg"})
+	_, err := m.Fetch(ctx, imagerequest.Source{Host: testHost, Path: "/photo.jpg"}, "", "")
 	if !errors.Is(err, context.Canceled) {
 		t.Errorf("Fetch() error = %v, want context.Canceled", err)
 	}