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/errorbus/errorbus.go

@@ -0,0 +1,21 @@
+package errorbus
+
+import (
+	"context"
+)
+
+// ErrorPublisher defines the interface for publishing errors to the error bus.
+type ErrorPublisher interface {
+	// Publish publishes an error to the error bus.
+	// The error will be logged and optionally reported to external services.
+	Publish(ctx context.Context, err error)
+}
+
+// ErrorContext provides additional context for errors.
+type ErrorContext struct {
+	RequestID string
+	UserID    string
+	Component string
+	Metadata  map[string]interface{}
+}
+