Refuse invalid settings at start, health check follows PORT (closes #31)

PORT (1 to 65535) and XDG_DATA_HOME (an absolute path) are now checked in
the config package, which is built before the database opens, so a bad
value stops the start with an error and a non-zero exit instead of leaving
the daemon running without HTTP or putting the database somewhere else.
entrypoint.sh refuses a MALLOC_ARENA_MAX that is not a positive whole
number, since glibc ignores a bad one silently. The HEALTHCHECK probes the
port PORT names. The README corrects the Linux state directory, lists
XDG_DATA_HOME, and adds a "Running under upaas" section.

Model: opus-5-5

TODO.md

@@ -23,6 +23,10 @@ runs make check on main.
 
 # Completed Steps
 
+- 2026-09-28: ready to run under upaas: a set but invalid `PORT`,
+  `XDG_DATA_HOME` or `MALLOC_ARENA_MAX` stops the start, the health
+  check follows `PORT`, README "Running under upaas" section (closes
+  #31)
 - 2026-09-22: realtime in-memory database statistics: counts seeded at
   startup and adjusted on every write, oldest/newest route timestamps via
   index-end lookups; `/api/v1/stats` no longer scans the tables (closes