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/imageproxy/concurrent_misses_internal_test.go

@@ -37,6 +37,7 @@ type heldFetcher struct {
 
 func (f *heldFetcher) Fetch(
 	ctx context.Context, source imagerequest.Source,
+	ifNoneMatch, ifModifiedSince string,
 ) (*httpfetcher.FetchResult, error) {
 	f.fetches.Add(1)
 
@@ -48,7 +49,7 @@ func (f *heldFetcher) Fetch(
 		return nil, ctx.Err()
 	}
 
-	return f.upstream.Fetch(ctx, source)
+	return f.upstream.Fetch(ctx, source, ifNoneMatch, ifModifiedSince)
 }
 
 // releaseFetches lets every held fetch, and every later one, go on.
@@ -489,7 +490,7 @@ func TestProxy_Get_WaiterGetsProcessingTimedOut(t *testing.T) {
 type panickingFetcher struct{}
 
 func (panickingFetcher) Fetch(
-	context.Context, imagerequest.Source,
+	context.Context, imagerequest.Source, string, string,
 ) (*httpfetcher.FetchResult, error) {
 	panic("upstream fetcher panicked")
 }