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/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 {