feat(epic1): implement core infrastructure (stories 1.1-1.5)

Implemented Epic 1 core kernel and infrastructure stories:

Story 1.1: Enhanced DI Container
- Added providers for database, health, metrics, and error bus
- Extended CoreModule to include all core services

Story 1.2: Database Layer with Ent ORM
- Created Ent schema for User, Role, Permission, AuditLog entities
- Implemented many-to-many relationships (User-Role, Role-Permission)
- Created database client wrapper with connection pooling
- Added database provider to DI container with migration support

Story 1.3: Health Monitoring and Metrics System
- Implemented health check registry and interface
- Added database health checker
- Created Prometheus metrics system with HTTP instrumentation
- Added health and metrics providers to DI container

Story 1.4: Error Handling and Error Bus
- Implemented channel-based error bus
- Created ErrorPublisher interface
- Added error bus provider with lifecycle management

Story 1.5: HTTP Server Foundation
- Created HTTP server with Gin framework
- Implemented comprehensive middleware stack:
  - Request ID generation
  - Structured logging
  - Panic recovery with error bus integration
  - Prometheus metrics collection
  - CORS support
- Registered core routes: /healthz, /ready, /metrics
- Integrated with FX lifecycle for graceful shutdown

All components are integrated via DI container and ready for use.

pkg/health/health.go

@@ -0,0 +1,34 @@
+package health
+
+import "context"
+
+// HealthChecker defines the interface for health checks.
+type HealthChecker interface {
+	// Check performs a health check and returns an error if unhealthy.
+	// Returns nil if the component is healthy.
+	Check(ctx context.Context) error
+}
+
+// Status represents the health status of a component.
+type Status string
+
+const (
+	// StatusHealthy indicates the component is healthy.
+	StatusHealthy Status = "healthy"
+	// StatusUnhealthy indicates the component is unhealthy.
+	StatusUnhealthy Status = "unhealthy"
+)
+
+// ComponentStatus represents the health status of a single component.
+type ComponentStatus struct {
+	Name   string `json:"name"`
+	Status Status `json:"status"`
+	Error  string `json:"error,omitempty"`
+}
+
+// HealthStatus represents the overall health status.
+type HealthStatus struct {
+	Status     Status            `json:"status"`
+	Components []ComponentStatus `json:"components,omitempty"`
+}
+