feat(channelserver): decouple channel servers for independent operation (#33)

Enable multiple Erupe instances to share a single PostgreSQL database
without destroying each other's state, fix existing data races in
cross-channel access, and lay groundwork for future distributed
channel server deployments.

Phase 1 — DB safety:
- Scope DELETE FROM servers/sign_sessions to this instance's server IDs
- Fix ci++ bug where failed channel start shifted subsequent IDs

Phase 2 — Fix data races in cross-channel access:
- Lock sessions map in FindSessionByCharID and DisconnectUser
- Lock stagesLock in handleMsgSysLockGlobalSema
- Snapshot sessions/stages under lock in TransitMessage types 1-4
- Lock channel when finding mail notification targets

Phase 3 — ChannelRegistry interface:
- Define ChannelRegistry interface with 7 cross-channel operations
- Implement LocalChannelRegistry with proper locking
- Add SessionSnapshot/StageSnapshot immutable copy types
- Delegate WorldcastMHF, FindSessionByCharID, DisconnectUser to Registry
- Migrate LockGlobalSema and guild mail handlers to use Registry
- Add comprehensive tests including concurrent access

Phase 4 — Per-channel enable/disable:
- Add Enabled *bool to EntranceChannelInfo (nil defaults to true)
- Skip disabled channels in startup loop, preserving ID stability
- Add IsEnabled() helper with backward-compatible default
- Update config.example.json with Enabled field

server/channelserver/handlers_guild_ops.go

@@ -304,11 +304,26 @@ func handleMsgMhfOperateGuildMember(s *Session, p mhfpacket.MHFPacket) {
 		doAckSimpleFail(s, pkt.AckHandle, make([]byte, 4))
 	} else {
 		_ = mail.Send(s, nil)
-		for _, channel := range s.server.Channels {
-			for _, session := range channel.sessions {
-				if session.charID == pkt.CharID {
-					SendMailNotification(s, &mail, session)
+		if s.server.Registry != nil {
+			s.server.Registry.NotifyMailToCharID(pkt.CharID, s, &mail)
+		} else {
+			// Fallback: find the target session under lock, then notify outside the lock.
+			var targetSession *Session
+			for _, channel := range s.server.Channels {
+				channel.Lock()
+				for _, session := range channel.sessions {
+					if session.charID == pkt.CharID {
+						targetSession = session
+						break
+					}
 				}
+				channel.Unlock()
+				if targetSession != nil {
+					break
+				}
+			}
+			if targetSession != nil {
+				SendMailNotification(s, &mail, targetSession)
 			}
 		}
 		doAckSimpleSucceed(s, pkt.AckHandle, make([]byte, 4))