feat: add multi-BU tenancy with per-user team scoping (Option B)

- Add bu_teams column to users table (migration + fresh schema)
- Create shared KNOWN_TEAMS constant and validateTeams helper
- Expose user teams in auth middleware, login, and /me responses
- Add bu_teams CRUD to user management routes with audit logging
- Make Ivanti FINDINGS_FILTERS configurable via IVANTI_BU_FILTER env var
- Add query-time team filtering to GET /findings and /findings/counts
- Update AuthContext with teams helpers and admin scope toggle
- Create AdminScopeToggle component (My Teams / All BUs)
- Scope ReportingPage findings fetch by user teams
- Scope CompliancePage team selector by user teams
- Scope ExportsPage findings exports by user teams
- Add BU teams multi-select to UserManagement create/edit forms
- Display team badges in user list table

backend/helpers/teams.js

@@ -0,0 +1,26 @@
+// Shared BU team constants and validation
+// Used by user management routes, auth middleware, and frontend-facing endpoints.
+
+const KNOWN_TEAMS = ['STEAM', 'ACCESS-ENG', 'ACCESS-OPS', 'INTELDEV'];
+
+/**
+ * Parse and validate a comma-separated teams string.
+ * @param {string} teamsString - Comma-separated team identifiers (e.g. 'STEAM,ACCESS-ENG')
+ * @returns {{ valid: boolean, teams: string[], invalid: string[] }}
+ */
+function validateTeams(teamsString) {
+    if (!teamsString || typeof teamsString !== 'string' || teamsString.trim() === '') {
+        return { valid: true, teams: [], invalid: [] };
+    }
+
+    const teams = teamsString.split(',').map(t => t.trim()).filter(Boolean);
+    const invalid = teams.filter(t => !KNOWN_TEAMS.includes(t));
+
+    return {
+        valid: invalid.length === 0,
+        teams,
+        invalid
+    };
+}
+
+module.exports = { KNOWN_TEAMS, validateTeams };