refactor: standardize bridge-agnostic terminology and remove unused kvstore functions

- Replace "Room" with "Channel" in bridge-agnostic contexts throughout codebase
- Update BridgeRoomID → BridgeChannelID in model structs and all references
- Change error messages to use consistent "Channel" terminology for user-facing text
- Update log keys: bridge_room_id → bridge_channel_id for consistency
- Clean up kvstore constants file by removing unused functions and constants:
  - Removed BuildXMPPUserKey, BuildMattermostUserKey, BuildGhostUserKey
  - Removed BuildXMPPEventPostKey, BuildXMPPReactionKey functions
  - Removed unused constants: KeyPrefixXMPPUser, KeyPrefixMattermostUser, etc.
  - Keep only actively used BuildChannelMapKey and ExtractIdentifierFromChannelMapKey
- Preserve XMPP-specific "Room" terminology in appropriate contexts (client methods, JIDs)

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

server/command/command.go

@@ -161,11 +161,11 @@ func (c *Handler) executeMapCommand(args *model.CommandArgs, fields []string) *m
 
 	// Create the mapping using BridgeManager
 	mappingReq := pluginModel.CreateChannelMappingRequest{
-		ChannelID:    channelID,
-		BridgeName:   "xmpp",
-		BridgeRoomID: roomJID,
-		UserID:       args.UserId,
-		TeamID:       args.TeamId,
+		ChannelID:       channelID,
+		BridgeName:      "xmpp",
+		BridgeChannelID: roomJID,
+		UserID:          args.UserId,
+		TeamID:          args.TeamId,
 	}
 
 	err = c.bridgeManager.CreateChannelMapping(mappingReq)
@@ -292,7 +292,7 @@ func (c *Handler) formatMappingError(operation, roomJID string, err error) *mode
 	case strings.Contains(errorMsg, "already mapped to channel"):
 		return &model.CommandResponse{
 			ResponseType: model.CommandResponseTypeEphemeral,
-			Text: fmt.Sprintf(`❌ **Room Already Mapped**
+			Text: fmt.Sprintf(`❌ **Channel Already Mapped**
 
 The XMPP room **%s** is already connected to another channel.
 
@@ -305,7 +305,7 @@ The XMPP room **%s** is already connected to another channel.
 	case strings.Contains(errorMsg, "does not exist"):
 		return &model.CommandResponse{
 			ResponseType: model.CommandResponseTypeEphemeral,
-			Text: fmt.Sprintf(`❌ **Room Not Found**
+			Text: fmt.Sprintf(`❌ **Channel Not Found**
 
 The XMPP room **%s** doesn't exist or isn't accessible.