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/transactions/TransactionsPage.tsx

@@ -12,7 +12,7 @@ import { IconPlus, IconEye, IconCheck, IconX, IconTrash, IconShieldCheck } from
 import { AttachmentPanel } from '../../components/attachments/AttachmentPanel';
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
 import api from '../../services/api';
-import { useIsReadOnly } from '../../stores/authStore';
+import { useCanEdit, CAPABILITIES } from '../../permissions';
 
 interface JournalEntryLine {
   id?: string;
@@ -49,7 +49,7 @@ export function TransactionsPage() {
   const [opened, { open, close }] = useDisclosure(false);
   const [viewId, setViewId] = useState<string | null>(null);
   const queryClient = useQueryClient();
-  const isReadOnly = useIsReadOnly();
+  const isReadOnly = !useCanEdit(CAPABILITIES.TRANSACTIONS_EDIT);
 
   const { data: entries = [], isLoading } = useQuery<JournalEntry[]>({
     queryKey: ['journal-entries'],