Fix uid/gidNumber allocation crash, add a configurable id floor (#44)

Reported: creating any user via the API failed with

  {"name":"InvalidSyntaxError","message":"gidNumber: value #0 invalid per syntax Code: 0x15"}

Root cause: addPosixGroup() computes the next gidNumber as
`Math.max(...groups.map(i => i.gidNumber)) + 1`. theta-env's
bootstrap.js creates the first admin via raw ldapadd with a hardcoded
uidNumber/gidNumber (10000) directly on the user entry, but never
creates a matching posixGroup entry -- so on a theta-env-bootstrapped
directory there are zero posixGroup entries, `Math.max()` on an empty
array is `-Infinity` in JS (not 0), and `-Infinity + 1` stringifies to
"-Infinity" -- an invalid LDAP integer, rejected by the directory. This
broke every single user creation, not just this one.

Separately: the reporter's intended scheme is for organically-created
users to start at uidNumber/gidNumber 1500, distinct from the
bootstrap admin's reserved 10000. Fixing the crash with a bare "floor
of 1500" alone wouldn't achieve that, since addPosixAccount's own
Math.max() would still find the admin's posixAccount entry (uidNumber
10000, found via a different, correctly-indexed search) and allocate
10001 for the next user.

Added a shared nextPosixId(entries, key) helper: takes the highest
existing value strictly below conf.ldap.uidGidReservedFloor (default
9000) plus one, or conf.ldap.uidGidMin (default 1500) if there are no
such entries. Ids at/above the reserved floor -- like the bootstrap
admin's 10000 -- are ignored entirely when computing the next
available number, so real users always start at 1500 and grow upward
regardless of the admin's reserved id.

Verified against a real theta-env deployment end to end:
- Reproduced the exact reported crash on a fresh bootstrap
- After the fix: first real user gets uidNumber/gidNumber "1500",
  second gets "1501" -- admin's 10000 never enters the calculation
- New unit tests (nodejs/tests/posix_id.test.js, no LDAP required):
  6/6 pass, covering the empty-array case, the reserved-floor
  exclusion, and the NaN-from-missing-value case
- npm test: 18/18 passing tests still pass (unchanged); the other 155
  failures are pre-existing/environmental (no LDAP server in this
  sandbox) -- confirmed via git stash before starting this fix

docs/configuration.md

@@ -29,6 +29,8 @@ raw strings otherwise.
 | `app_ldap__url=ldap://host:389` | `conf.ldap.url` | string |
 | `app_ldap__bindPassword=secret` | `conf.ldap.bindPassword` | string |
 | `app_ldap__userBase=ou=people,dc=…` | `conf.ldap.userBase` | string |
+| `app_ldap__uidGidMin=1500` | `conf.ldap.uidGidMin` | number (new-user id floor) |
+| `app_ldap__uidGidReservedFloor=9000` | `conf.ldap.uidGidReservedFloor` | number (ids at/above this are ignored when allocating) |
 | `app_oauth__jwtSecret=...` | `conf.oauth.jwtSecret` | string |
 | `app_oauth__issuer=https://sso.example.com` | `conf.oauth.issuer` | string |
 | `app_oauth__token_lifetime__access_token=3600` | `conf.oauth.token_lifetime.access_token` | number |