Use pg_class estimation for outgoing and dead letter table counts

[jeremydmiller]

Summary

Fixes #2377

Replaces expensive select count(*) queries on the wolverine_outgoing_envelopes and wolverine_dead_letters tables with pg_class reltuples-based estimation in PostgresqlMessageStore.FetchCountsAsync().

This matches the approach already used for the partitioned incoming table. The estimation uses PostgreSQL catalog statistics (pg_class.reltuples and pg_relation_size) which are virtually free compared to sequential scans. Falls back to exact count(*) only when the table has physical data but reltuples hasn't been updated by VACUUM/ANALYZE.

Before

select count(*) from schema.wolverine_outgoing_envelopes   -- full sequential scan
select count(*) from schema.wolverine_dead_letters          -- full sequential scan

After

-- Fast catalog lookup, no table scan
select (case when c.reltuples < 0 then 0 ... end)::bigint as estimated_count, ...
from pg_catalog.pg_class c ...
where c.relname = 'wolverine_outgoing_envelopes';

Test plan

• Build succeeds
• 15 durability tests pass
• 3 PostgreSQL-specific tests pass (10 failures are pre-existing SQL Server connection issues)

🤖 Generated with Claude Code