Periodically sync remote gates instead of only loading once at startup

Gates were only ever loaded from the shared database at plugin enable
or /sg reload, so a server that's already running never found out
about a gate another backend created afterward - the row existed in
MySQL, but nothing told this JVM to look again. Reported as "no other
gates on network 'main'" despite both gates being visibly present in
the database.

Added GateManager.refreshRemoteGates(), scheduled every
cross-server.refresh-seconds (default 30) whenever cross-server is
enabled: it re-queries storage off the main thread, then merges only
OTHER servers' gates into gatesById on the main thread - adding new
ones, removing deleted ones, and never touching this server's own
gates or any live open/connected/outgoing runtime state.

stargate-paper/src/main/resources/config.yml

@@ -33,6 +33,12 @@ storage:
 # share gate data.
 cross-server:
   enabled: false
+  # How often (in seconds) this server checks the shared database for gates other
+  # servers have created, updated, or removed since it last looked. Without this, a
+  # server that's already running never finds out about a gate another backend made
+  # after it booted - the row exists, but nothing prompts a reload. Doesn't touch
+  # this server's own gates or any live dial/open state.
+  refresh-seconds: 30
 
 gate:
   # Any of these materials count as the ring's frame (and count as a valid chevron