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 every time
pixa stores for them two days back (database rows, metadata files and
file times), 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