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.

Makefile

@@ -110,6 +110,27 @@ generate:
 	@echo "Running code generation..."
 	$(GO) generate ./...
 
+generate-proto:
+	@echo "Generating gRPC code from proto files..."
+	@if ! command -v protoc > /dev/null; then \
+		echo "protoc not found. Install Protocol Buffers compiler."; \
+		exit 1; \
+	fi
+	@if ! command -v protoc-gen-go > /dev/null; then \
+		echo "protoc-gen-go not found. Install with: go install google.golang.org/protobuf/cmd/protoc-gen-go@latest"; \
+		exit 1; \
+	fi
+	@if ! command -v protoc-gen-go-grpc > /dev/null; then \
+		echo "protoc-gen-go-grpc not found. Install with: go install google.golang.org/grpc/cmd/protoc-gen-go-grpc@latest"; \
+		exit 1; \
+	fi
+	@mkdir -p api/proto/generated
+	@protoc --go_out=api/proto/generated --go_opt=paths=source_relative \
+		--go-grpc_out=api/proto/generated --go-grpc_opt=paths=source_relative \
+		--proto_path=api/proto \
+		api/proto/*.proto
+	@echo "gRPC code generation complete"
+
 verify: fmt-check lint test
 	@echo "Verification complete"