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.example.json

@@ -219,34 +219,34 @@
       {
         "Name": "Newbie", "Description": "", "IP": "", "Type": 3, "Recommended": 2, "AllowedClientFlags": 0,
         "Channels": [
-          { "Port": 54001, "MaxPlayers": 100 },
-          { "Port": 54002, "MaxPlayers": 100 }
+          { "Port": 54001, "MaxPlayers": 100, "Enabled": true },
+          { "Port": 54002, "MaxPlayers": 100, "Enabled": true }
         ]
       }, {
         "Name": "Normal", "Description": "", "IP": "", "Type": 1, "Recommended": 0, "AllowedClientFlags": 0,
         "Channels": [
-          { "Port": 54003, "MaxPlayers": 100 },
-          { "Port": 54004, "MaxPlayers": 100 }
+          { "Port": 54003, "MaxPlayers": 100, "Enabled": true },
+          { "Port": 54004, "MaxPlayers": 100, "Enabled": true }
         ]
       }, {
         "Name": "Cities", "Description": "", "IP": "", "Type": 2, "Recommended": 0, "AllowedClientFlags": 0,
         "Channels": [
-          { "Port": 54005, "MaxPlayers": 100 }
+          { "Port": 54005, "MaxPlayers": 100, "Enabled": true }
         ]
       }, {
         "Name": "Tavern", "Description": "", "IP": "", "Type": 4, "Recommended": 0, "AllowedClientFlags": 0,
         "Channels": [
-          { "Port": 54006, "MaxPlayers": 100 }
+          { "Port": 54006, "MaxPlayers": 100, "Enabled": true }
         ]
       }, {
         "Name": "Return", "Description": "", "IP": "", "Type": 5, "Recommended": 0, "AllowedClientFlags": 0,
         "Channels": [
-          { "Port": 54007, "MaxPlayers": 100 }
+          { "Port": 54007, "MaxPlayers": 100, "Enabled": true }
         ]
       }, {
         "Name": "MezFes", "Description": "", "IP": "", "Type": 6, "Recommended": 6, "AllowedClientFlags": 0,
         "Channels": [
-          { "Port": 54008, "MaxPlayers": 100 }
+          { "Port": 54008, "MaxPlayers": 100, "Enabled": true }
         ]
       }
     ]