feat(db/m7-user-mgmt): migration 0022 + audit helper

Schema for user management U1: display_name on users; user_invites;
registration_settings singleton (default 'invite_only'); audit_log.
Plus the internal/audit package centralizing the action-name
vocabulary and JSON metadata marshaling so handlers don't repeat
boilerplate.

Race-safe first-admin uses a query-shape primitive
(CreateUserFirstAdminRace's WHERE NOT EXISTS subquery) rather than
a schema-level constraint. Concurrent empty-state registrations
both see 'no users yet' and both insert as admin — fine, having
two admins from the start is benign; what matters is at-least-one.
users.username uniqueness arbitrates if the two callers picked the
same username.

CreateUser signature gains display_name (nullable); existing
bootstrap call sites pass nil. The audit package declares the full
U1+U2+U3 action vocabulary upfront so subsequent slices are purely
additive on the caller side.

Tests cover audit Write with + without metadata and that every
declared action constant persists correctly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/db/migrations/0022_user_management.down.sql

@@ -0,0 +1,9 @@
+-- Reverses migration 0022. Drop tables in reverse dependency order.
+-- This will fail if any rows in audit_log or user_invites reference
+-- users that were deleted with ON DELETE SET NULL semantics; those
+-- references will already be NULL so the drops should succeed cleanly.
+
+DROP TABLE IF EXISTS audit_log;
+DROP TABLE IF EXISTS user_invites;
+DROP TABLE IF EXISTS registration_settings;
+ALTER TABLE users DROP COLUMN IF EXISTS display_name;