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.

internal/metrics/metrics.go

@@ -0,0 +1,97 @@
+package metrics
+
+import (
+	"net/http"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+// Metrics holds all Prometheus metrics.
+type Metrics struct {
+	httpRequestDuration *prometheus.HistogramVec
+	httpRequestTotal   *prometheus.CounterVec
+	httpErrorsTotal    *prometheus.CounterVec
+	registry           *prometheus.Registry
+}
+
+// NewMetrics creates a new metrics registry with all metrics.
+func NewMetrics() *Metrics {
+	registry := prometheus.NewRegistry()
+
+	m := &Metrics{
+		registry: registry,
+		httpRequestDuration: prometheus.NewHistogramVec(
+			prometheus.HistogramOpts{
+				Name:    "http_request_duration_seconds",
+				Help:    "HTTP request duration in seconds",
+				Buckets: prometheus.DefBuckets,
+			},
+			[]string{"method", "path", "status"},
+		),
+		httpRequestTotal: prometheus.NewCounterVec(
+			prometheus.CounterOpts{
+				Name: "http_requests_total",
+				Help: "Total number of HTTP requests",
+			},
+			[]string{"method", "path", "status"},
+		),
+		httpErrorsTotal: prometheus.NewCounterVec(
+			prometheus.CounterOpts{
+				Name: "http_errors_total",
+				Help: "Total number of HTTP errors",
+			},
+			[]string{"method", "path", "status"},
+		),
+	}
+
+	// Register all metrics
+	registry.MustRegister(m.httpRequestDuration)
+	registry.MustRegister(m.httpRequestTotal)
+	registry.MustRegister(m.httpErrorsTotal)
+
+	return m
+}
+
+// HTTPMiddleware returns a Gin middleware for collecting HTTP metrics.
+func (m *Metrics) HTTPMiddleware() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		start := time.Now()
+
+		// Process request
+		c.Next()
+
+		// Calculate duration
+		duration := time.Since(start).Seconds()
+
+		// Get request details
+		method := c.Request.Method
+		path := c.FullPath()
+		if path == "" {
+			path = c.Request.URL.Path
+		}
+		status := c.Writer.Status()
+
+		// Record metrics
+		m.httpRequestDuration.WithLabelValues(method, path, http.StatusText(status)).Observe(duration)
+		m.httpRequestTotal.WithLabelValues(method, path, http.StatusText(status)).Inc()
+
+		// Record errors (4xx and 5xx)
+		if status >= 400 {
+			m.httpErrorsTotal.WithLabelValues(method, path, http.StatusText(status)).Inc()
+		}
+	}
+}
+
+// Handler returns an HTTP handler for the /metrics endpoint.
+func (m *Metrics) Handler() http.Handler {
+	return promhttp.HandlerFor(m.registry, promhttp.HandlerOpts{})
+}
+
+// Registry returns the Prometheus registry.
+func (m *Metrics) Registry() *prometheus.Registry {
+	return m.registry
+}
+