feat: add flexible capability-based RBAC with per-tenant customization

Introduces a capability layer on top of existing roles that controls
feature visibility and access. Capabilities follow an area.feature.action
taxonomy (~35 capabilities) with sensible defaults per role. Tenant admins
can customize via grant/revoke overrides stored in org settings JSONB.

Key changes:
- Add vice_president role to DB schema
- Backend: capability constants, resolution logic, CapabilityGuard (global),
  @RequireCapability decorator on all 16 tenant controllers
- Frontend: permission hooks (useCanEdit, useHasCapability), CapabilityGate
  component, sidebar filtering by capability, all 17 pages migrated from
  useIsReadOnly to capability-based checks
- New admin UI: /settings/permissions matrix page for per-tenant role
  customization with grant/revoke delta model
- GET /organizations/my-capabilities endpoint for capability refresh
- Validation of permissionOverrides in settings updates

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/budgets/BudgetsPage.tsx

@@ -8,7 +8,7 @@ import { IconDeviceFloppy, IconInfoCircle, IconPencil, IconX, IconArrowRight } f
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
 import { useNavigate } from 'react-router-dom';
 import api from '../../services/api';
-import { useIsReadOnly } from '../../stores/authStore';
+import { useCanEdit, CAPABILITIES } from '../../permissions';
 import { usePreferencesStore } from '../../stores/preferencesStore';
 
 interface BudgetLine {
@@ -40,7 +40,7 @@ export function BudgetsPage() {
   const [editData, setEditData] = useState<BudgetLine[] | null>(null); // null = not editing
   const queryClient = useQueryClient();
   const navigate = useNavigate();
-  const isReadOnly = useIsReadOnly();
+  const isReadOnly = !useCanEdit(CAPABILITIES.FINANCIALS_BUDGETS_EDIT);
   const isDark = usePreferencesStore((s) => s.colorScheme) === 'dark';
   const stickyBg = isDark ? 'var(--mantine-color-dark-7)' : 'white';
   const stickyBorder = isDark ? 'var(--mantine-color-dark-4)' : '#e9ecef';