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/queries/users.sql

@@ -1,6 +1,28 @@
 -- name: CreateUser :one
-INSERT INTO users (username, password_hash, api_token, is_admin)
-VALUES ($1, $2, $3, $4)
+INSERT INTO users (username, password_hash, api_token, is_admin, display_name)
+VALUES ($1, $2, $3, $4, $5)
+RETURNING *;
+
+-- name: CreateUserFirstAdminRace :one
+-- Inserts a new user; sets is_admin=true ONLY when no users currently
+-- exist. The (SELECT NOT EXISTS ...) subquery is evaluated at INSERT
+-- time within the same statement, so the result reflects committed
+-- state at that moment.
+--
+-- Race semantics: two concurrent calls in an empty-users state may
+-- both observe "no users" and both insert with is_admin=true. That's
+-- benign — having two admins from the gate is fine; what matters is
+-- that there's AT LEAST one. If the two calls happened to share the
+-- same username, the unique constraint on users.username arbitrates
+-- and the second caller's INSERT fails with a unique violation. The
+-- caller (registration handler) can retry as a regular non-admin in
+-- that case (or surface a "username taken" error to the user).
+INSERT INTO users (username, password_hash, api_token, is_admin, display_name)
+VALUES (
+    $1, $2, $3,
+    (SELECT NOT EXISTS (SELECT 1 FROM users)),
+    $4
+)
 RETURNING *;
 
 -- name: GetUserByUsername :one