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.