Task: Fix codename with sequence-based unique suffix

Replace the advisory-lock + NOT-EXISTS approach with a sequence-based unique suffix so codenames are unique even under batch insert.

• ores_utility_to_base26_fn(bigint) encodes sequence values into lowercase base-26 strings (satisfies codename regex constraints).
• ores_refdata_party_codename_seq sequence provides unique MVCC-independent values.
• ores_refdata_parties_insert_fn trigger appends the base-26 suffix to whimsical names; guarantees uniqueness per row in a batch.
• party_repository::write(vector) reinstated as a proper batch insert (the per-row workaround is no longer needed).

Captured during execution; cleared into the parent story on close.

The advisory lock from the previous task was ineffective for batch inserts; the sequence-based approach is correct by construction.

Codenames unique under batch insert; batch insert restored.