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/auth/bootstrap.go

@@ -59,6 +59,7 @@ func Bootstrap(ctx context.Context, pool *pgxpool.Pool, cfg config.AdminBootstra
 		PasswordHash: string(hash),
 		ApiToken:     apiToken,
 		IsAdmin:      true,
+		DisplayName:  nil,
 	})
 	if err != nil {
 		return fmt.Errorf("auth: insert admin: %w", err)