Delete guest accounts left idle for five days

The app had no cleanup of any kind: in multi-user mode every first visit
mints a users row, so the demo has been accumulating one permanent account
per visitor along with everything they generated.

cleanup.py sweeps guests idle for AIDND_GUEST_RETENTION_DAYS (default 5),
once at startup and then every few hours. Startup is the load-bearing
trigger — the free tier sleeps after ~15 minutes, so a long timer rarely
gets to fire.

Idle is COALESCE(last_seen_at, created_at), not last_seen_at: _touch only
writes that column hourly, and a guest minted by /auth/me has it NULL until
its second request, so the simpler query would have deleted brand-new
visitors mid-session.

It's one Core DELETE rather than db.delete(user), which would SELECT every
adventure, action and memory into Python purely to delete them — the same
egress pattern as the 189x fix. Every FK from users down is ON DELETE
CASCADE, so the database does the whole graph and returns a count.

The filter requires is_guest AND email IS NULL, so registered users (who
upgrade in place) and local mode's implicit user are both out of reach, and
is_public is output-only so a guest can never own content another user can
see. Session cookies have no expiry and can outlive a swept row; that path
401s and the frontend's existing retry re-mints a session.

Guests are told: /auth/me serves guest_retention_days and the signup modal
states the window, sourced from the server so it can't drift from what is
enforced.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015CYEJKobJ2Re4Dv7qUoSA7

backend/.env.example

@@ -66,6 +66,16 @@ AIDND_DEMO_TURNS_PER_DAY=
 # Local (single-user) installs are always treated as power users.
 AIDND_POWER_USERS=
 
+# --- Guest retention (only active when AIDND_MULTI_USER=1) ---
+# Every first visit mints a guest account, so a public demo collects one row
+# per visitor. A guest with no activity for this many days is deleted along
+# with its scenarios, adventures and actions. Registered accounts are never
+# touched. Default: 5. Set 0 to keep guests forever.
+AIDND_GUEST_RETENTION_DAYS=
+# How often a running process re-checks. The sweep also runs once at startup,
+# which is what actually fires on hosts that sleep. Default: 6
+AIDND_CLEANUP_INTERVAL_HOURS=
+
 # The AI endpoint/API key/model are NOT env vars — they are configured at
 # runtime in the app's Settings page and stored (encrypted) in the database.
 #