refactor: Align Epic 0 & Epic 1 with true microservices architecture

Refactor core kernel and infrastructure to support true microservices
architecture where services are independently deployable.

Phase 1: Core Kernel Cleanup
- Remove database provider from CoreModule (services create their own)
- Update ProvideHealthRegistry to not depend on database
- Add schema support to database client (NewClientWithSchema)
- Update main entry point to remove database dependency
- Core kernel now provides only: config, logger, error bus, health, metrics, tracer, service registry

Phase 2: Service Registry Implementation
- Create ServiceRegistry interface (pkg/registry/registry.go)
- Implement Consul registry (internal/registry/consul/consul.go)
- Add Consul dependency (github.com/hashicorp/consul/api)
- Add registry configuration to config/default.yaml
- Add ProvideServiceRegistry() to DI container

Phase 3: Service Client Interfaces
- Create service client interfaces:
  - pkg/services/auth.go - AuthServiceClient
  - pkg/services/identity.go - IdentityServiceClient
  - pkg/services/authz.go - AuthzServiceClient
  - pkg/services/audit.go - AuditServiceClient
- Create ServiceClientFactory (internal/client/factory.go)
- Create stub gRPC client implementations (internal/client/grpc/)
- Add ProvideServiceClientFactory() to DI container

Phase 4: gRPC Service Definitions
- Create proto files for all core services:
  - api/proto/auth.proto
  - api/proto/identity.proto
  - api/proto/authz.proto
  - api/proto/audit.proto
- Add generate-proto target to Makefile

Phase 5: API Gateway Implementation
- Create API Gateway service entry point (cmd/api-gateway/main.go)
- Create Gateway implementation (services/gateway/gateway.go)
- Add gateway configuration to config/default.yaml
- Gateway registers with Consul and routes requests to backend services

All code compiles successfully. Core services (Auth, Identity, Authz, Audit)
will be implemented in Epic 2 using these foundations.

config/default.yaml

@@ -24,3 +24,30 @@ tracing:
   service_name: "platform"
   service_version: "1.0.0"
   otlp_endpoint: ""
+
+registry:
+  type: consul
+  consul:
+    address: "localhost:8500"
+    datacenter: "dc1"
+    scheme: "http"
+    health_check:
+      interval: "10s"
+      timeout: "3s"
+      deregister_after: "30s"
+      http: "/healthz"
+
+gateway:
+  port: 8080
+  host: "0.0.0.0"
+  routes:
+    - path: "/api/v1/auth/**"
+      service: "auth-service"
+      auth_required: false
+    - path: "/api/v1/users/**"
+      service: "identity-service"
+      auth_required: true
+  cors:
+    allowed_origins: ["*"]
+    allowed_methods: ["GET", "POST", "PUT", "DELETE", "PATCH"]
+    allowed_headers: ["Authorization", "Content-Type"]