Tell upaas operators to set the cache size limit (closes #251)

"Running under upaas" listed PIXA_CACHE_MAX_BYTES as optional. Under
upaas the volume is a directory on the host's filesystem. The default,
75% of the sum of that filesystem's free space and what the cache
holds, and at least 500 MiB, lets one pixa take three quarters or more
of the space the host and every other app have left, and all of it
when that is 500 MiB or less. The section now says to set it and why.
The default is unchanged.

Model: opus-5-5

TODO.md

@@ -30,6 +30,13 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-15 "Running under upaas" in `README.md` tells the operator to set
+  `PIXA_CACHE_MAX_BYTES` (closes #251): it listed the variable as optional, but
+  under upaas the volume is a directory on the host's filesystem. The default,
+  75% of the sum of that filesystem's free space and what the cache holds, and
+  at least 500 MiB, let one pixa take three quarters or more of the space the
+  host and every other app had left, and all of it when that was 500 MiB or
+  less. The section now says so. The default is unchanged.
 - 2026-10-15 a `:` in the source path or query no longer moves where the path
   ends and the query begins (closes #247): the signed data and the cache key
   joined host, path and query with `:` and no escaping, so a URL signed for