Remove the unused cache lifetime setting (closes #69)

CacheConfig.CacheTTL was set to DefaultCacheTTL (24 hours) and never
read, so cached images never expired. Remove the field, the constant
and the line that set it. Cached source and transformed images stay
until eviction removes them to keep the cache within cache_max_bytes.

A new test caches a source and a transformed image, sets back two days
fetched_at in source_metadata, fetched_at and last_accessed_at in
source_content, created_at and last_accessed_at in variant_content, the
fetched_at in the source's metadata file, the created_at in the .meta
file, and the access and modification times of those two files and of
the two image files, and checks that both are still served from the
cache without a new upstream fetch.

Existing tests lose only the CacheTTL line of their cache config.

Model: opus-5-5

internal/imgcache/cache.go

@@ -35,7 +35,6 @@ const metaCacheSize = 10000
 // CacheConfig holds cache configuration.
 type CacheConfig struct {
 	StateDir    string
-	CacheTTL    time.Duration
 	NegativeTTL time.Duration
 
 	// MaxBytes is the disk cache size limit in bytes that eviction