Add cleanup, minor fixes

.env.example

@@ -40,6 +40,11 @@ COOKIE_SECURE=true
 SESSION_SHORT_DURATION=24h
 SESSION_LONG_DURATION=720h
 
+# How often the backend purges expired sessions, spent/expired invites and
+# never-activated users (Go duration string). Set to 0 (or a negative value) to
+# disable the background cleanup.
+CLEANUP_INTERVAL=1h
+
 # --- jitsi -----------------------------------------------------------------
 # Shared secret and app id must match the Jitsi (JaaS) deployment that signs
 # and verifies room tokens. JITSI_SECRET has no default and must be set for