Feat v0.6.15 user display audit

Add batch user resolve endpoint, sw.users SDK module, and migrate all
surfaces to show display_name instead of UUIDs. Canonical fallback chain:
display_name → username → "Unknown".

- GET /api/v1/users/resolve?ids=... (max 100, map response)
- sw.users.resolve(), resolveMany(), displayName() with 60s cache
- Chat participants resolved from users table, not snapshot
- Admin users/teams/groups/team-admin show display_name
- Chat-core participants.display_name column deprecated
- 5 new handler tests, OpenAPI spec updated

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/store/sqlite/user.go

@@ -262,6 +262,47 @@ func (s *UserStore) SearchActive(ctx context.Context, excludeUserID, query strin
 	return results, rows.Err()
 }
 
+// ResolveByIDs returns lightweight identity records for the given user IDs.
+func (s *UserStore) ResolveByIDs(ctx context.Context, ids []string) ([]store.UserSearchResult, error) {
+	if len(ids) == 0 {
+		return []store.UserSearchResult{}, nil
+	}
+	if len(ids) > 100 {
+		ids = ids[:100]
+	}
+
+	// Build ?,?,... placeholders
+	placeholders := make([]string, len(ids))
+	args := make([]interface{}, len(ids))
+	for i, id := range ids {
+		placeholders[i] = "?"
+		args[i] = id
+	}
+
+	q := `SELECT id, username, COALESCE(display_name, '') AS display_name,
+	             COALESCE(handle, '') AS handle, COALESCE(avatar_url, '') AS avatar_url
+	      FROM users WHERE id IN (` + strings.Join(placeholders, ",") + `)`
+
+	rows, err := DB.QueryContext(ctx, q, args...)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var results []store.UserSearchResult
+	for rows.Next() {
+		var u store.UserSearchResult
+		if err := rows.Scan(&u.ID, &u.Username, &u.DisplayName, &u.Handle, &u.AvatarURL); err != nil {
+			continue
+		}
+		results = append(results, u)
+	}
+	if results == nil {
+		results = []store.UserSearchResult{}
+	}
+	return results, rows.Err()
+}
+
 // ── CS2 additions ─────────────────────────────────────────────
 
 func (s *UserStore) MergeSettings(ctx context.Context, userID string, patch []byte) error {