feat: add multi-tenancy foundation (directory-per-tenant)

Implements Phase 1-2 of multi-tenancy support using a directory-per-tenant
strategy that preserves existing file-based persistence.

Key changes:
- Add MultiTenantPersistence manager for org-scoped config routing
- Add TenantMiddleware for X-Pulse-Org-ID header extraction and context propagation
- Add MultiTenantMonitor for per-tenant monitor lifecycle management
- Refactor handlers (ConfigHandlers, AlertHandlers, AIHandlers, etc.) to be
  context-aware with getConfig(ctx)/getMonitor(ctx) helpers
- Add Organization model for future tenant metadata
- Update server and router to wire multi-tenant components

All handlers maintain backward compatibility via legacy field fallbacks
for single-tenant deployments using the "default" org.

mock.env

@@ -0,0 +1,9 @@
+# Mock Mode Configuration
+PULSE_MOCK_MODE=true
+PULSE_MOCK_NODES=7
+PULSE_MOCK_VMS_PER_NODE=5
+PULSE_MOCK_LXCS_PER_NODE=8
+PULSE_MOCK_DOCKER_HOSTS=3
+PULSE_MOCK_DOCKER_CONTAINERS=12
+PULSE_MOCK_RANDOM_METRICS=true
+PULSE_MOCK_STOPPED_PERCENT=20