fix(db): add conversation order index

[FGadvancer]

Summary

• Use a stable, deterministic order when loading the conversation list.
• Add a SQLite partial expression index so the query does not need temporary sorting files on read-only filesystems.
• Create the index during initial database setup and SDK version migration.

Testing

• go test ./pkg/db -run TestConversationListSplitUsesVisibleOrderIndex -count=1 -v