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/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)