fix: Lazy-load JWKS in JWT validator + add unit tests

Changes:
- Make JWT validator lazy-load JWKS on first use (not on init)
- Thread-safe JWKS loading with mutex
- Fixes test failures (JWKS 404 was panicking on NewValidator)
- Add unit tests for JWT validation logic

Tests now pass:
  ✅ Check permissions (sqs:read, sqs:write, wildcard)
  ✅ Reject empty/invalid/malformed tokens
  ✅ Handle missing permissions claim

All 100% passing with no external dependencies.

internal/auth/jwt.go

@@ -3,6 +3,7 @@ package auth
 import (
 	"context"
 	"fmt"
+	"sync"
 	"time"
 
 	"github.com/MicahParks/keyfunc/v2"
@@ -13,33 +14,48 @@ import (
 type Validator struct {
 	issuer   string
 	audience string
+	jwksURL  string
 	jwks     *keyfunc.JWKS
+	mu       sync.Mutex
 }
 
 // NewValidator creates a new JWT validator for a service.
+// JWKS fetching is lazy (deferred until first validation).
 func NewValidator(issuer, audience, jwksURL string) *Validator {
-	// Create JWKS from URL with automatic refresh
+	return &Validator{
+		issuer:   issuer,
+		audience: audience,
+		jwksURL:  jwksURL,
+		jwks:     nil, // Lazy-loaded on first use
+	}
+}
+
+// ensureJWKS fetches JWKS on first use (lazy initialization, thread-safe).
+func (v *Validator) ensureJWKS() error {
+	v.mu.Lock()
+	defer v.mu.Unlock()
+
+	if v.jwks != nil {
+		return nil
+	}
+
 	options := keyfunc.Options{
 		Ctx:              context.Background(),
 		RefreshInterval:  15 * time.Minute,
 		RefreshRateLimit: 5 * time.Minute,
 		RefreshTimeout:   10 * time.Second,
 		RefreshErrorHandler: func(err error) {
-			// Log refresh errors but don't fail
-			fmt.Printf("JWKS refresh error for %s: %v\n", issuer, err)
+			fmt.Printf("JWKS refresh error for %s: %v\n", v.issuer, err)
 		},
 	}
 
-	jwks, err := keyfunc.Get(jwksURL, options)
+	jwks, err := keyfunc.Get(v.jwksURL, options)
 	if err != nil {
-		panic(fmt.Sprintf("failed to fetch JWKS from %s: %v", jwksURL, err))
+		return fmt.Errorf("failed to fetch JWKS from %s: %v", v.jwksURL, err)
 	}
 
-	return &Validator{
-		issuer:   issuer,
-		audience: audience,
-		jwks:     jwks,
-	}
+	v.jwks = jwks
+	return nil
 }
 
 // ValidateBearerToken extracts and validates the Bearer token from Authorization header.
@@ -57,6 +73,11 @@ func (v *Validator) ValidateBearerToken(authHeader string) (jwt.MapClaims, error
 		return nil, fmt.Errorf("invalid Authorization header format")
 	}
 
+	// Ensure JWKS is loaded (lazy)
+	if err := v.ensureJWKS(); err != nil {
+		return nil, err
+	}
+
 	// Parse and validate
 	claims := jwt.MapClaims{}
 	token, err := jwt.ParseWithClaims(tokenString, claims, v.jwks.Keyfunc)