Give pixad a fixed uid and gid 65532 (closes #151)

adduser took the first free uid, 1000, and the entrypoint gives a
bind-mounted /var/lib/pixa to pixad, so on the host a person's login
account ended up owning pixa's database and cache. The image now creates
the pixad group with gid 65532 and the pixad user with uid 65532, which
host login and system accounts do not use. The first-run step of
"Running under upaas" in README.md names the uid and gid.

Model: opus-5-5

TODO.md

@@ -30,6 +30,11 @@ exhaustion
 
 # Completed Steps
 
+- 2026-09-29 fixed uid and gid for `pixad` (closes #151): the image creates the
+  `pixad` group with gid 65532 and the `pixad` user with uid 65532, instead of
+  the first free uid 1000, so a bind-mounted `/var/lib/pixa` given to `pixad`
+  is not owned on the host by a person's login account; the first-run step of
+  "Running under upaas" in `README.md` names the uid and gid.
 - 2026-09-29 `max-age` never outlives an expiring URL (closes #63): both image
   routes build `Cache-Control` from the request's `Expires`, which an encrypted
   URL's expiry now fills too; `max-age` is one year, or the whole seconds left