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

Cached images were served until eviction, so upstream changes were
missed.

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; 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 for checking a
variant of an evicted source.

A variant file's modification time is its source's check time when it
was made or renewed. Hits read it without a database call. The ETag
includes it, so new bytes get a new ETag.

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)
 	}