Implement Multi-Tenant Architecture Support

## Overview
Add multi-tenancy support to enable multiple organizations to use the platform with complete isolation.

## Business Value
Multi-tenancy is essential for SaaS deployment and enables serving multiple customers on a single instance.

## Current State
- Single-tenant architecture
- Organization concept exists but no isolation
- Cannot support multiple customers

## Subtasks
- [ ] Design tenant isolation strategy
- [ ] Implement tenant context middleware
- [ ] Add tenant data isolation
- [ ] Implement resource quotas
- [ ] Add tenant management
- [ ] Create tenant admin UI
- [ ] Test tenant isolation

## Implementation Steps
1. Add tenant context to all requests
2. Implement data filtering by tenant
3. Add quota enforcement
4. Create tenant management
5. Test isolation

## Acceptance Criteria
- [ ] Tenants are completely isolated
- [ ] No data leakage between tenants
- [ ] Resource quotas are enforced
- [ ] Tenant management works
- [ ] Isolation is tested

## Estimated Effort
- Hours: 40-60
- Complexity: High