Add Admin

Add or promote a user to the admin team.

2026-06-09 — Resilience pass. Previously every DB-side failure surfaced as a generic 500 with no JSON body, so the FE could only show "Something went wrong on our side." This now:

• Normalises the email and validates the shape early.
• Returns 404 with an ACTIONABLE message when the user has never signed in (the most common cause of "Failed").
• Wraps the INSERT in a savepoint so a column-drift or ON-CONFLICT race doesn't abort the whole transaction.
• Treats _log_action failure as non-fatal — the admin grant is the primary side effect; the audit row is best-effort.