R6(b): reconcile docs with code — restore guest-reaper mention

Pass (a) removed a stale "(reaping abandoned guest rows is deferred — TODO-3)"
note from ARCHITECTURE §3, but guest reaping is implemented (the background
reaper, BACKEND_GUEST_REAP_INTERVAL / BACKEND_GUEST_RETENTION, covered by
inttest). State the current behaviour instead.

A full section-by-section review of ARCHITECTURE / FUNCTIONAL (+_ru) / TESTING /
UI_DESIGN against the code found no other drift — each R-phase baked its own docs,
and FUNCTIONAL/TESTING already describe the reaper correctly.

docs/ARCHITECTURE.md

@@ -168,8 +168,11 @@ arrive from a platform rather than completing a mandatory registration).
   row is a technical necessity (the `sessions` and `game_players` foreign keys
   require one, the same way the robot pool is durable), not a profile: no
   friends, statistics or history are kept for it, and it is restricted to
-  auto-match. Platform and email users are auto-provisioned **durable** accounts
-  with an identity.
+  auto-match. A background **guest reaper** deletes an abandoned guest — flagged
+  `is_guest`, holding no game seat, older than `BACKEND_GUEST_RETENTION` — on a
+  `BACKEND_GUEST_REAP_INTERVAL` sweep, so transient guest rows do not accumulate.
+  Platform and email users are auto-provisioned **durable** accounts with an
+  identity.
 
 ## 4. Accounts, identities, linking & merge