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

The default cache_max_bytes 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; newCacheConfig in the handlers
turns the disk cache off only for an explicit 0, and is tested for an
omitted, a zero and a positive value.

Model: opus-5-5

config.example.yml

@@ -128,8 +128,9 @@ access_control_allow_origin: "*"
 
 # Maximum disk cache size in bytes. Explicit values are used exactly as
 # given; 0 disables the disk cache entirely (every request fetches and
-# processes uncached). When omitted, the default is 75% of the free
-# space on the filesystem containing <state_dir>/cache/ at startup,
+# processes uncached). When omitted, the default is 75% of the sum of
+# the free space on the filesystem containing <state_dir>/cache/ and
+# the bytes of images the cache already holds, worked out at startup,
 # with a minimum of 500 MiB.
 # cache_max_bytes: 10737418240