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

config/config.go

@@ -297,6 +297,15 @@ type EntranceChannelInfo struct {
 	Port           uint16
 	MaxPlayers     uint16
 	CurrentPlayers uint16
+	Enabled        *bool // nil defaults to true for backward compatibility
+}
+
+// IsEnabled returns whether this channel is enabled. Defaults to true if Enabled is nil.
+func (c *EntranceChannelInfo) IsEnabled() bool {
+	if c.Enabled == nil {
+		return true
+	}
+	return *c.Enabled
 }
 
 var ErupeConfig *Config