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

No files, entry points, or tests are named. Start by defining the tenant isolation strategy and tracing requests through the tenant context middleware, then map the data isolation, quota, tenant management, and tenant admin UI work. Done means tenant data is isolated, quotas and management work, and isolation tests show no data leakage.

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