Implement token blacklist for JWT logout support

- TokenBlacklistService tracks invalidated tokens using Caffeine cache.
- AuthController adds tokens to blacklist on logout.
- JwtAuthenticationFilter blocks blacklisted tokens during authentication.

src/main/java/com/skycrate/backend/skycrateBackend/security/JwtAuthenticationFilter.java

@@ -20,10 +20,14 @@ public class JwtAuthenticationFilter extends OncePerRequestFilter {
 
     private final JwtService jwtService;
     private final UserRepository userRepository;
+    private final TokenBlacklistService tokenBlacklistService;
 
-    public JwtAuthenticationFilter(JwtService jwtService, UserRepository userRepository) {
+    public JwtAuthenticationFilter(JwtService jwtService,
+                                   UserRepository userRepository,
+                                   TokenBlacklistService tokenBlacklistService) {
         this.jwtService = jwtService;
         this.userRepository = userRepository;
+        this.tokenBlacklistService = tokenBlacklistService;
     }
 
     @Override
@@ -42,6 +46,14 @@ public class JwtAuthenticationFilter extends OncePerRequestFilter {
         }
 
         jwt = authHeader.substring(7);
+
+        // Check if token is blacklisted
+        if (tokenBlacklistService.isTokenBlacklisted(jwt)) {
+            response.setStatus(HttpServletResponse.SC_UNAUTHORIZED);
+            response.getWriter().write("Token has been blacklisted");
+            return;
+        }
+
         try {
             userEmail = jwtService.extractUsername(jwt);
         } catch (Exception e) {

src/main/java/com/skycrate/backend/skycrateBackend/security/TokenBlacklistService.java

@@ -0,0 +1,27 @@
+package com.skycrate.backend.skycrateBackend.security;
+
+import com.github.benmanes.caffeine.cache.Cache;
+import com.github.benmanes.caffeine.cache.Caffeine;
+import org.springframework.stereotype.Service;
+
+import java.util.concurrent.TimeUnit;
+
+@Service
+public class TokenBlacklistService {
+
+    private final Cache<String, Boolean> blacklistCache;
+
+    public TokenBlacklistService() {
+        this.blacklistCache = Caffeine.newBuilder()
+                .expireAfterWrite(1, TimeUnit.HOURS)
+                .build();
+    }
+
+    public void blacklistToken(String token) {
+        blacklistCache.put(token, true);
+    }
+
+    public boolean isTokenBlacklisted(String token) {
+        return blacklistCache.getIfPresent(token) != null;
+    }
+}