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/providers/providers.go

@@ -83,6 +83,7 @@ func newCache(lc fx.Lifecycle, params cacheParams) (*imgcache.Cache, error) {
 	cache, err := imgcache.NewCache(db, imgcache.Config{
 		StateDir:    cfg.StateDir,
 		NegativeTTL: imgcache.DefaultNegativeTTL,
+		CacheTTL:    imgcache.DefaultCacheTTL,
 		MaxBytes:    maxBytes,
 		Logger:      params.Logger.Get(),
 	}, params.MetricsRegistry)