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/services/identity.go

@@ -0,0 +1,62 @@
+// Package services provides service client interfaces for inter-service communication.
+package services
+
+import (
+	"context"
+)
+
+// IdentityServiceClient is the interface for communicating with the Identity Service.
+type IdentityServiceClient interface {
+	// GetUser retrieves a user by ID.
+	GetUser(ctx context.Context, id string) (*User, error)
+
+	// GetUserByEmail retrieves a user by email address.
+	GetUserByEmail(ctx context.Context, email string) (*User, error)
+
+	// CreateUser creates a new user.
+	CreateUser(ctx context.Context, user *CreateUserRequest) (*User, error)
+
+	// UpdateUser updates an existing user.
+	UpdateUser(ctx context.Context, id string, user *UpdateUserRequest) (*User, error)
+
+	// DeleteUser deletes a user.
+	DeleteUser(ctx context.Context, id string) error
+
+	// VerifyEmail verifies a user's email address using a verification token.
+	VerifyEmail(ctx context.Context, token string) error
+
+	// RequestPasswordReset requests a password reset token.
+	RequestPasswordReset(ctx context.Context, email string) error
+
+	// ResetPassword resets a user's password using a reset token.
+	ResetPassword(ctx context.Context, token, newPassword string) error
+}
+
+// User represents a user in the system.
+type User struct {
+	ID            string `json:"id"`
+	Email         string `json:"email"`
+	Username      string `json:"username"`
+	FirstName     string `json:"first_name"`
+	LastName      string `json:"last_name"`
+	EmailVerified bool   `json:"email_verified"`
+	CreatedAt     int64  `json:"created_at"`
+	UpdatedAt     int64  `json:"updated_at"`
+}
+
+// CreateUserRequest contains the data needed to create a new user.
+type CreateUserRequest struct {
+	Email     string `json:"email"`
+	Username  string `json:"username"`
+	Password  string `json:"password"`
+	FirstName string `json:"first_name"`
+	LastName  string `json:"last_name"`
+}
+
+// UpdateUserRequest contains the data needed to update a user.
+type UpdateUserRequest struct {
+	Email     *string `json:"email,omitempty"`
+	Username  *string `json:"username,omitempty"`
+	FirstName *string `json:"first_name,omitempty"`
+	LastName  *string `json:"last_name,omitempty"`
+}