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.

pkg/registry/registry.go

@@ -0,0 +1,41 @@
+// Package registry provides service registry interface for service discovery.
+package registry
+
+import (
+	"context"
+)
+
+// ServiceRegistry is the interface for service discovery and registration.
+type ServiceRegistry interface {
+	// Register registers a service instance with the registry.
+	Register(ctx context.Context, service *ServiceInstance) error
+
+	// Deregister removes a service instance from the registry.
+	Deregister(ctx context.Context, serviceID string) error
+
+	// Discover returns all healthy instances of a service.
+	Discover(ctx context.Context, serviceName string) ([]*ServiceInstance, error)
+
+	// Watch returns a channel that receives updates when service instances change.
+	Watch(ctx context.Context, serviceName string) (<-chan []*ServiceInstance, error)
+
+	// Health returns the health status of a service instance.
+	Health(ctx context.Context, serviceID string) (*HealthStatus, error)
+}
+
+// ServiceInstance represents a service instance in the registry.
+type ServiceInstance struct {
+	ID       string            // Unique instance ID
+	Name     string            // Service name (e.g., "auth-service", "identity-service")
+	Address  string            // Service address (IP or hostname)
+	Port     int               // Service port
+	Tags     []string          // Service tags for filtering
+	Metadata map[string]string // Additional metadata
+}
+
+// HealthStatus represents the health status of a service instance.
+type HealthStatus struct {
+	ServiceID string // Service instance ID
+	Status    string // Health status: "healthy", "unhealthy", "critical"
+	Message   string // Optional status message
+}