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

TODO.md

@@ -30,6 +30,11 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-13 the unused cache lifetime is removed (closes #69):
+  `CacheConfig.CacheTTL`, which pixad set to `DefaultCacheTTL` (24 hours), was
+  never read, so cached images never expired. The field and the constant are
+  removed. Cached source and transformed images stay until eviction removes them
+  to keep the cache within `cache_max_bytes`.
 - 2026-10-13 SVG sources are refused, and `orig` is never saved as JPEG in place
   of the source's own format (closes #68): the upstream fetch no longer accepts
   `image/svg+xml`, which libvips would render. Both image routes answer 415 for