feat(db): add operator account lifecycle storage

[n30nex]

What this PR does

Add the account-storage foundation for #60: an additive accounts table, active-name uniqueness, generated sqlc queries/mocks and a narrow AccountStore implementation. It is independent of pending authentication/API work and exposes no endpoint.

Names are trimmed, case-sensitive and limited to 128 Unicode characters without control characters. Deactivation preserves the row and timestamp; a later account can reuse the inactive name. Duplicate active names, absent IDs and repeated deactivation have distinct errors. A row-locking query makes concurrent deactivation return one success and one already-inactive outcome. The migration is retryable and creates no account data.

Type of change

• New feature
• Tests

Checklist

• Build, format, vet and full Go tests pass
• Pure validation/mapping tests added
• Migration SQL and regenerated sqlc/mocks included
• Swagger regeneration produces no change for this storage-only PR
• No new dependency
• CONTRIBUTING.md read

Testing notes

Native Pi 5/PostgreSQL 16 tests apply the migration twice and cover empty lists, Unicode/trimmed names, validation, case sensitivity, missing IDs, deactivation timestamp preservation, name reuse and deterministic lists. Eight concurrent creates yield one winner; repeated concurrent deactivation rounds yield one success each. Tests create and remove their own schema in the isolated test database. Full local/native checks and focused local race checks pass.

CI and CodeQL pass. The combined preview applied 034 with an empty accounts table, a valid partial unique index, and an unchanged schema dump for all pre-existing objects. The exact preceding binary also started and served its existing private test endpoints with the additive schema present; rollback retains the table and migration record. Account HTTP operations are the dependent draft #149.

AI-assisted implementation/testing under the contributor's standing authorization; submitted for review without claiming a new specific human diff review.

[n30nex]

@MrAlders0n please review this change with your Claude agent as part of the September 13 Beacon review batch. The current candidate is 88aeae1fe4da; validation and Pi evidence are in the PR description.

Please check migration 034, active-name uniqueness, soft deactivation and preserved records. This is storage for account API #149, and these records are not login principals. Issue #60 remains open for the full admin scope.