feat(auth): implement master key authentication system with child key issuance

Add admin and master authentication layers with JWT support. Replace direct
key creation with hierarchical master/child key system. Update database
schema to support master accounts with configurable limits and epoch-based
key revocation. Add health check endpoint with system status monitoring.

BREAKING CHANGE: Removed direct POST /keys endpoint in favor of master-based
key issuance through /v1/tokens. Database migration requires dropping old User
table and creating Master table with new relationships.

internal/service/health.go

@@ -0,0 +1,56 @@
+package service
+
+import (
+	"context"
+	"time"
+
+	"github.com/redis/go-redis/v9"
+	"gorm.io/gorm"
+)
+
+type HealthCheckService struct {
+	db  *gorm.DB
+	rdb *redis.Client
+}
+
+func NewHealthCheckService(db *gorm.DB, rdb *redis.Client) *HealthCheckService {
+	return &HealthCheckService{
+		db:  db,
+		rdb: rdb,
+	}
+}
+
+type HealthStatus struct {
+	Status   string `json:"status"`
+	Database string `json:"database"`
+	Redis    string `json:"redis"`
+	Uptime   string `json:"uptime"`
+}
+
+var startTime = time.Now()
+
+func (s *HealthCheckService) Check(ctx context.Context) HealthStatus {
+	status := HealthStatus{
+		Status:   "ok",
+		Database: "up",
+		Redis:    "up",
+		Uptime:   time.Since(startTime).String(),
+	}
+
+	sqlDB, err := s.db.DB()
+	if err != nil || sqlDB.Ping() != nil {
+		status.Database = "down"
+		status.Status = "degraded"
+	}
+
+	if s.rdb.Ping(ctx).Err() != nil {
+		status.Redis = "down"
+		status.Status = "degraded"
+	}
+
+	if status.Database == "down" && status.Redis == "down" {
+		status.Status = "down"
+	}
+
+	return status
+}