docs(docstrings): add docstrings to public method interfaces (#25)

- Resolves #18

Reviewed-on: #25

src/main/java/com/ddf/vodsystem/services/UserService.java

@@ -44,10 +44,21 @@ public class UserService {
         this.jwtService = jwtService;
     }
 
+    /**
+     * Looks up a user by their database ID.
+     *
+     * @param userId the ID of the user to find
+     * @return the matching {@link User}, or empty if none exists
+     */
     public Optional<User> getUserById(Long userId) {
         return userRepository.findById(userId);
     }
 
+    /**
+     * Returns the user backing the current Spring Security authentication, if authenticated.
+     *
+     * @return the authenticated {@link User}, or empty if no user is authenticated
+     */
     public Optional<User> getLoggedInUser() {
         Authentication auth = SecurityContextHolder.getContext().getAuthentication();
         if (auth != null && auth.isAuthenticated() && auth.getPrincipal() instanceof User user) {
@@ -56,6 +67,13 @@ public class UserService {
         return Optional.empty();
     }
 
+    /**
+     * Verifies a Google ID token, creating or updating the corresponding user, and issues a session JWT.
+     *
+     * @param idToken the Google ID token to verify
+     * @return a signed JWT for the authenticated user
+     * @throws NotAuthenticated if the token is invalid or its subject is missing
+     */
     public String login(String idToken) {
         GoogleIdToken googleIdToken = getGoogleIdToken(idToken);
         String googleId = googleIdToken.getPayload().getSubject();
@@ -70,6 +88,12 @@ public class UserService {
         return jwtService.generateToken(user.getId());
     }
 
+    /**
+     * Looks up a user by their stream key.
+     *
+     * @param streamKey the stream key to match
+     * @return the matching {@link User}, or empty if none exists
+     */
     public Optional<User> getUserByStreamKey(String streamKey) {
         return userRepository.findByStreamKey(streamKey);
     }