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/httpfetcher/mock.go

@@ -24,9 +24,10 @@ func NewMock(fsys fs.FS) *MockFetcher {
 	return &MockFetcher{fs: fsys}
 }
 
-// Fetch retrieves the source's content from the mock filesystem.
+// Fetch retrieves the source's content from the mock filesystem. It ignores
+// ifNoneMatch and ifModifiedSince, and always sends the content.
 func (m *MockFetcher) Fetch(
-	ctx context.Context, source imagerequest.Source,
+	ctx context.Context, source imagerequest.Source, _, _ string,
 ) (*FetchResult, error) {
 	// Check context cancellation
 	select {