Function catch_up_dm_conversation 

pub fn catch_up_dm_conversation(
    conn: &mut Connection,
    room_id: &str,
    messages: &[LegacyDmMessageImport],
    receipts: &[LegacyDmReadReceipt],
) -> Result<DmMigrationCounts, MatrixStoreError>

A catch-up pass for an ALREADY migrated room (P15) — the binary crate’s own matrix_migration module calls this both from a live legacy DM send’s bridge (routes::dm::create_message, right after its own insert commits) and from a boot pass over a conversation room_by_legacy_dm_id already finds a room for. messages (already filtered by the caller to exclude anything already in legacy_dm_message_map, ordered ascending by legacy_message_id) lands as new org.example.legacy_dm timeline events plus their legacy_dm_message_map rows; then each of receipts advances that reader’s m.read receipt — never moved backwards ([upsert_receipt_in_tx]’s own guarantee, the SAME one a live /receipt call gets) — all in ONE transaction. Reuses the exact per-event helpers migrate_dm_conversation itself uses, so a caught-up room’s rows are indistinguishable from ones a fresh migration (or a live Matrix send) would have produced.

Each receipt.up_to_legacy_message_id is resolved via legacy_dm_message_event_id against the FULL map (this transaction’s own just-inserted rows are visible to it too, same connection) — not just this call’s own messages — because the caller’s own read-position query considers EVERY read message in the conversation, including ones a PRIOR pass already imported (a message read only after it was already bridged must still move the receipt here; manager review, P15). A target this catch-up’s own messages did not just insert AND that is not yet in the map at all (should not happen — a receipt only ever targets a message that exists) is skipped rather than erroring, same defensive posture migrate_dm_conversation’s own receipt loop takes.