feat(channel): pass conversation type through to agent gateway and persist in route

Propagate conversation type (direct/group/thread) from channel adapters
all the way to the agent prompt. Store conversation_type on bot_channel_routes
so the bot knows whether a message originates from a p2p chat, group, or thread.

Schema changes are folded into the 0001 init migration (destructive update).

internal/db/sqlc/models.go

@@ -49,6 +49,7 @@ type BotChannelRoute struct {
 	ChannelConfigID        pgtype.UUID        `json:"channel_config_id"`
 	ExternalConversationID string             `json:"external_conversation_id"`
 	ExternalThreadID       pgtype.Text        `json:"external_thread_id"`
+	ConversationType       pgtype.Text        `json:"conversation_type"`
 	DefaultReplyTarget     pgtype.Text        `json:"default_reply_target"`
 	Metadata               []byte             `json:"metadata"`
 	CreatedAt              pgtype.Timestamptz `json:"created_at"`