[BUG] upsertMany misses NIP-01 tie-break when timestamps are equal #536

Describe the bug

upsert() and upsertMany() currently behave differently for replaceable events that share the same created_at.

upsert() applies the NIP-01 fallback (lower lexical event_id wins), but upsertMany() only updates on strictly newer timestamps.

In batch paths (including import), this can keep the wrong winner for equal-timestamp conflicts.

To Reproduce

1. Create two valid replaceable events with the same dedup key (pubkey + kind or pubkey + kind + d) and same created_at.
2. Ensure the second event has a lower lexical id.
3. Persist via a batch path that uses upsertMany() (for example, import flow).
4. Observe that the stored winner may not follow the NIP-01 tie-break rule.

Expected behavior
When created_at is equal, relay should keep the event with lower lexical event_id.

Actual behavior
upsertMany() uses only this condition:

"events"."event_created_at" < "excluded"."event_created_at"

There is no equal-timestamp fallback on event_id.

Suggested fix
Mirror the upsert() conflict condition in upsertMany():

• events.event_created_at < excluded.event_created_at
• OR (events.event_created_at = excluded.event_created_at AND events.event_id > excluded.event_id)