Turn the media cache on by default !12

Stacked on !11; retarget to main once that merges.

Built-in defaults: cache.enabled true, lifetime 1w, max-size 200 MB, memcache off. A config that omits the block gets a working media cache; one that sets enabled to false still turns it off. The existing writability check makes an instance that cannot cache fail at startup rather than silently proxy every image. SETUP.md updated; one new test pins the defaults.

Closes #12