feat: Multi-tenant support with namespace isolation

[initcron]

Summary

Implement multi-tenant support allowing multiple teams/organizations to share a gateway with proper isolation.

Parent Epic

Part of #1 - Production Kubernetes & Container Support

Use Cases

• Platform teams providing MCP gateway as a service
• SaaS deployments with customer isolation
• Enterprise multi-team environments
• Development/staging/production separation

Architecture

┌──────────────────────────────────────────────────────────┐
│                    MCP Gateway                            │
├──────────────────────────────────────────────────────────┤
│  ┌─────────────┐  ┌─────────────┐  ┌─────────────┐       │
│  │  Tenant A   │  │  Tenant B   │  │  Tenant C   │       │
│  │  (team-a)   │  │  (team-b)   │  │  (team-c)   │       │
│  ├─────────────┤  ├─────────────┤  ├─────────────┤       │
│  │ namespace:  │  │ namespace:  │  │ namespace:  │       │
│  │ mcp-team-a  │  │ mcp-team-b  │  │ mcp-team-c  │       │
│  │ catalog:    │  │ catalog:    │  │ catalog:    │       │
│  │ team-a.yaml │  │ team-b.yaml │  │ team-c.yaml │       │
│  │ quota: 5 srv│  │ quota: 10srv│  │ quota: 3 srv│       │
│  └─────────────┘  └─────────────┘  └─────────────┘       │
└──────────────────────────────────────────────────────────┘

Tenant Configuration

# tenants.yaml
tenants:
  - id: team-a
    display_name: Team Alpha
    namespace: mcp-team-a
    catalog_file: catalogs/team-a.yaml
    api_keys:
      - key_hash: "sha256:abc123..."
        name: "production"
      - key_hash: "sha256:def456..."
        name: "development"
    quotas:
      max_servers: 5
      max_concurrent_requests: 100
    
  - id: team-b
    display_name: Team Beta
    namespace: mcp-team-b
    catalog_file: catalogs/team-b.yaml
    api_keys:
      - key_hash: "sha256:xyz789..."
    quotas:
      max_servers: 10

API Changes

Tenant Identification

# Via header
X-Tenant-ID: team-a

# Via path prefix
GET /tenant/team-a/servers

# Via API key (automatic lookup)
Authorization: Bearer <team-a-api-key>

Tenant-Scoped Endpoints

GET    /tenant/{tenant_id}/servers
POST   /tenant/{tenant_id}/servers/{server_id}/mcp
GET    /tenant/{tenant_id}/metrics

Features Required

• Tenant configuration file (tenants.yaml)
• Per-tenant catalogs
• Per-tenant API keys with hashed storage
• Namespace isolation for K8s runtimes
• Resource quotas per tenant
• Tenant-scoped metrics
• Hot reload of tenant configuration

Acceptance Criteria

• Multiple tenants can be configured
• Each tenant sees only their servers
• API keys map to specific tenants
• K8s resources created in tenant namespace
• Quotas are enforced
• Metrics are labeled by tenant

Security Considerations

• API keys stored as secure hashes
• Namespace RBAC prevents cross-tenant access
• Rate limiting per tenant
• Audit logging of tenant actions

References

• Kubernetes Namespaces
• Multi-tenancy patterns