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/bridge/xmpp/message_handler.go

@@ -163,4 +163,4 @@ func (r *xmppUserResolver) GetDisplayName(externalUserID string) string {
 
 	// Fallback to the full ID
 	return externalUserID
-}
+}