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/model/message.go

@@ -30,8 +30,8 @@ type BridgeMessage struct {
 	ThreadID  string    // Thread/reply ID (if applicable)
 
 	// Routing hints
-	TargetBridges []string        // Which bridges should receive this
-	Metadata      map[string]any  // Bridge-specific metadata
+	TargetBridges []string       // Which bridges should receive this
+	Metadata      map[string]any // Bridge-specific metadata
 }
 
 // DirectionalMessage wraps a BridgeMessage with direction information
@@ -85,4 +85,4 @@ type MessageHandler interface {
 
 	// GetSupportedMessageTypes returns the message types this handler supports
 	GetSupportedMessageTypes() []string
-}
+}