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/middleware/auth.go

@@ -0,0 +1,63 @@
+package middleware
+
+import (
+	"net/http"
+	"strings"
+
+	"github.com/ez-api/ez-api/internal/service"
+	"github.com/gin-gonic/gin"
+)
+
+func AdminAuthMiddleware(adminService *service.AdminService) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		authHeader := c.GetHeader("Authorization")
+		if authHeader == "" {
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "authorization header required"})
+			c.Abort()
+			return
+		}
+
+		parts := strings.Split(authHeader, " ")
+		if len(parts) != 2 || parts[0] != "Bearer" {
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "invalid authorization header format"})
+			c.Abort()
+			return
+		}
+
+		if !adminService.ValidateToken(parts[1]) {
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "invalid admin token"})
+			c.Abort()
+			return
+		}
+
+		c.Next()
+	}
+}
+
+func MasterAuthMiddleware(masterService *service.MasterService) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		authHeader := c.GetHeader("Authorization")
+		if authHeader == "" {
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "authorization header required"})
+			c.Abort()
+			return
+		}
+
+		parts := strings.Split(authHeader, " ")
+		if len(parts) != 2 || parts[0] != "Bearer" {
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "invalid authorization header format"})
+			c.Abort()
+			return
+		}
+
+		master, err := masterService.ValidateMasterKey(parts[1])
+		if err != nil {
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "invalid master key"})
+			c.Abort()
+			return
+		}
+
+		c.Set("master", master)
+		c.Next()
+	}
+}