Cardano Updates

epoch is now a view (migration-2-0049), not a table: document it as the UNION
of epoch_finalized + epoch_current, and add the new epoch_current view and
epoch_finalized / epoch_sync_enabled tables. Also correct the header note - the
schema-doc generator (BaseSchema.hs / gen-schema-docs) was removed in the
Persistent->Hasql rewrite, so the file is now maintained manually.

test(uniqueness): assert pool_stat is unique per (pool_hash_id, epoch_no)

pool_stat should hold at most one row per pool per epoch. Add a
check_unique assertion in the POOLS section of test-uniqueness.sql
so duplicate (pool_hash_id, epoch_no) rows are caught.

Covers #1986, #1901, #1987.

the epoch_current was triggered for all entries

feat(offchain): add --allow-private-offchain-urls flag

The pool_relay encoder serialized the unsigned Word16 port as a signed
int2, so relay ports above 32767 wrapped to negative values
(port - 65536). The pool_relay.port SQL column is int4, so the wrapped
value was stored verbatim.

- Encode the port as int4 to match the column and preserve the full
  0-65535 range.
- Add migration-2-0049 to repair already-stored rows by adding 65536 to
  every negative port (no-op on a clean or freshly synced database).
- Add a cardano-db IO regression test that inserts a relay with port
  41950 and asserts it round-trips (fails with the old int2 encoder,
  reading back -23586).
- Give the chain-gen test relay a port > 32767 so the end-to-end pool
  path also exercises the full range.

2127-Replace epoch table with maintained finalized + live current views