feat(auth): RequireUser middleware (cookie or bearer)

Resolves /api/* callers from session cookie first, Authorization
bearer second. Touches last_seen on success for future active-
sessions UI. Adds GetUserByID query used by the middleware.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/db/dbq/users.sql.go

@@ -74,6 +74,25 @@ func (q *Queries) GetUserByAPIToken(ctx context.Context, apiToken string) (User,
 	return i, err
 }
 
+const getUserByID = `-- name: GetUserByID :one
+SELECT id, username, password_hash, api_token, is_admin, created_at, subsonic_password FROM users WHERE id = $1
+`
+
+func (q *Queries) GetUserByID(ctx context.Context, id pgtype.UUID) (User, error) {
+	row := q.db.QueryRow(ctx, getUserByID, id)
+	var i User
+	err := row.Scan(
+		&i.ID,
+		&i.Username,
+		&i.PasswordHash,
+		&i.ApiToken,
+		&i.IsAdmin,
+		&i.CreatedAt,
+		&i.SubsonicPassword,
+	)
+	return i, err
+}
+
 const getUserByUsername = `-- name: GetUserByUsername :one
 SELECT id, username, password_hash, api_token, is_admin, created_at, subsonic_password FROM users WHERE username = $1
 `