Count what the cache holds in the default cache_max_bytes (closes #184)

The default limit was 75% of the space free at startup. The cache's own
files are not free space, so a fuller cache got a smaller limit after a
restart and eviction then deleted most of it. The default is now 75% of
the sum of the free space and what the cache already holds by its own
size accounting, at least 500 MiB. The cache works it out when it
opens, after the database is open, so the computation and its tests
moved from internal/config to internal/imgcache; the config only
records whether cache_max_bytes was set.

Model: opus-5-5

TODO.md

@@ -29,6 +29,14 @@ P2: security: referer blacklist
 
 # Completed Steps
 
+- 2026-10-04 the default `cache_max_bytes` no longer shrinks as the cache fills
+  (closes #184): for an omitted key, the cache works out the limit when it
+  opens, after the database is open, as 75% of the sum of the free space on the
+  filesystem containing `<state_dir>/cache/` and what the cache already holds by
+  its own size accounting, at least 500 MiB, so a cache filled to its limit
+  keeps that limit across a restart. The computation and its tests moved from
+  `internal/config` to `internal/imgcache`; the config only records whether the
+  key was set.
 - 2026-10-04 deployment guide and example Caddy config (closes #89):
   "Deployment" in `README.md` says what the reverse proxy in front of pixa must
   do (terminate TLS; pass `Host`, `Origin` and `Referer` on unchanged; set