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/util/hash.go

@@ -0,0 +1,12 @@
+package util
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+)
+
+func HashToken(token string) string {
+	hasher := sha256.New()
+	hasher.Write([]byte(token))
+	return hex.EncodeToString(hasher.Sum(nil))
+}