Speed up get_unconfirmed_messages with denormalized first_confirmed_at

[odesenfans]

Summary

• Replace the NOT EXISTS anti-join against message_confirmations + chain_txs in get_unconfirmed_messages with a direct check on messages.first_confirmed_at IS NULL. The column is already trigger-maintained (migration 0049), so no schema change is needed.
• Drop the now-unused chain parameter and update the two callers (chains/ethereum.py, chains/nuls2.py) accordingly.

Performance

Measured against a production-sized dataset (~22k unconfirmed signed messages):

Before (anti-join): ~10s
After (denorm column, using existing ix_messages_first_confirmed): ~300ms cold, ~50ms warm. No new index needed.

Limit  (cost=26259.44..26260.69 rows=500 width=1275) (actual time=48.696..48.796 rows=500 loops=1)
  ->  Sort  (Sort Key: reception_time, Sort Method: top-N heapsort  Memory: 738kB)
        ->  Bitmap Heap Scan on messages (Recheck Cond: first_confirmed_at IS NULL, Filter: signature IS NOT NULL)
              ->  Bitmap Index Scan on ix_messages_first_confirmed (Index Cond: first_confirmed_at IS NULL)
Execution Time: 48.856 ms

Semantic note

The previous implementation filtered confirmations by chain (e.g. chain=Chain.ETH for the ETH packer). first_confirmed_at is set by a confirmation on any chain. In production only ETH confirmations are recorded, so the behavior is equivalent today. If multi-chain confirmations are reintroduced, the unconfirmed query will need to reconsider this.

Test plan

• hatch run testing:test tests/db/test_messages.py::test_get_unconfirmed_messages tests/db/test_messages.py::test_get_unconfirmed_messages_trusted_messages -v
• Spot-check ETH and NULS2 packer paths still call get_unconfirmed_messages without chain=.

[foxpatch-aleph]

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[foxpatch-aleph]

Clean, well-motivated refactor replacing a slow anti-join with a direct IS NULL check on the trigger-maintained first_confirmed_at column. Removes the now-unnecessary chain parameter and handles both callers. The semantic change (any chain's confirmation now counts) is actually more correct than the old chain-filtered behavior.

src/aleph/db/accessors/messages.py (line 421): Excellent simplification. The old branching logic between chain=None and chain= was a source of complexity for a feature (per-chain unconfirmed queries) that had no real use case.