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

"Running under upaas" in README.md listed PIXA_CACHE_MAX_BYTES as
optional. Under upaas the volume is a directory on the host's
filesystem, and the default limit is 75% of the sum of that
filesystem's free space and what the cache holds, at least 500 MiB, so
one pixa can take three quarters or more of the space every app has
left, and all of it when that is 500 MiB or less. The section now tells
the operator to set the variable and says what the default does there.
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 the request metrics are labelled by route, not by path (closes
   #254): their `handler` label was each request's path, so every new path, a 404
   or an encrypted URL, added series to the Prometheus registry that were never