feat: create respository and controllers for creating and viewing invitations

backend/src/main/java/online/wesal/wesal/controller/InvitationController.java

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+package online.wesal.wesal.controller;
+
+import io.swagger.v3.oas.annotations.Operation;
+import io.swagger.v3.oas.annotations.tags.Tag;
+import jakarta.validation.Valid;
+import online.wesal.wesal.dto.ApiResponse;
+import online.wesal.wesal.dto.CreateInvitationRequest;
+import online.wesal.wesal.dto.InvitationResponse;
+import online.wesal.wesal.service.InvitationService;
+import org.springframework.validation.BindingResult;
+import org.springframework.validation.FieldError;
+
+import java.util.stream.Collectors;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.http.ResponseEntity;
+import org.springframework.security.core.Authentication;
+import org.springframework.web.bind.annotation.*;
+
+@RestController
+@RequestMapping("/invitations")
+@CrossOrigin(origins = "*")
+@Tag(name = "Invitations", description = "Invitation management endpoints")
+public class InvitationController {
+    
+    @Autowired
+    private InvitationService invitationService;
+    
+    @PostMapping(value = "/create", consumes = "application/json", produces = "application/json")
+    @Operation(summary = "Create invitation", description = "Create a new invitation")
+    public ResponseEntity<ApiResponse<InvitationResponse>> createInvitation(
+            @Valid @RequestBody CreateInvitationRequest request,
+            BindingResult bindingResult,
+            Authentication authentication) {
+        
+        if (bindingResult.hasErrors()) {
+            String errorMessage = bindingResult.getFieldErrors().stream()
+                .map(error -> {
+                    String field = error.getField();
+                    if ("title".equals(field)) return "Title is required";
+                    if ("description".equals(field)) return "Description is required";
+                    if ("maxParticipants".equals(field)) return "Maximum participants must be a positive number";
+                    if ("tagId".equals(field)) return "Tag ID is required";
+                    return error.getDefaultMessage();
+                })
+                .collect(Collectors.joining(", "));
+            return ResponseEntity.badRequest().body(ApiResponse.error(errorMessage));
+        }
+        
+        try {
+            String userEmail = authentication.getName();
+            InvitationResponse response = invitationService.createInvitation(request, userEmail);
+            return ResponseEntity.ok(ApiResponse.success(response));
+        } catch (RuntimeException e) {
+            String message;
+            if (e.getMessage().contains("User not found")) {
+                message = "Authentication error. Please log in again.";
+            } else if (e.getMessage().contains("Tag not found")) {
+                message = "The selected tag does not exist.";
+            } else {
+                message = "Something went wrong.. We're sorry but try again later";
+            }
+            return ResponseEntity.badRequest().body(ApiResponse.error(message));
+        }
+    }
+    
+    @GetMapping("/get")
+    @Operation(summary = "Get invitation", description = "Get invitation by ID")
+    public ResponseEntity<ApiResponse<InvitationResponse>> getInvitation(@RequestParam Long id) {
+        if (id == null || id <= 0) {
+            return ResponseEntity.badRequest().body(ApiResponse.error("Valid invitation ID is required"));
+        }
+        
+        try {
+            return invitationService.getInvitationById(id)
+                    .map(invitation -> ResponseEntity.ok(ApiResponse.success(invitation)))
+                    .orElse(ResponseEntity.status(404).body(ApiResponse.error("Invitation not found")));
+        } catch (Exception e) {
+            return ResponseEntity.status(500).body(ApiResponse.error("Something went wrong.. We're sorry but try again later"));
+        }
+    }
+}

backend/src/main/java/online/wesal/wesal/repository/AttendeeRepository.java

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+package online.wesal.wesal.repository;
+
+import online.wesal.wesal.entity.Attendee;
+import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.stereotype.Repository;
+
+@Repository
+public interface AttendeeRepository extends JpaRepository<Attendee, Long> {
+}

backend/src/main/java/online/wesal/wesal/repository/InvitationRepository.java

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+package online.wesal.wesal.repository;
+
+import online.wesal.wesal.entity.Invitation;
+import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.stereotype.Repository;
+
+@Repository
+public interface InvitationRepository extends JpaRepository<Invitation, Long> {
+}

backend/src/main/java/online/wesal/wesal/repository/TagRepository.java

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+package online.wesal.wesal.repository;
+
+import online.wesal.wesal.entity.Tag;
+import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.stereotype.Repository;
+
+@Repository
+public interface TagRepository extends JpaRepository<Tag, Long> {
+}

backend/src/main/java/online/wesal/wesal/service/InvitationService.java

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+package online.wesal.wesal.service;
+
+import online.wesal.wesal.dto.CreateInvitationRequest;
+import online.wesal.wesal.dto.InvitationResponse;
+import online.wesal.wesal.entity.Invitation;
+import online.wesal.wesal.entity.Tag;
+import online.wesal.wesal.entity.User;
+import online.wesal.wesal.repository.InvitationRepository;
+import online.wesal.wesal.repository.TagRepository;
+import online.wesal.wesal.repository.UserRepository;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+import java.util.Optional;
+
+@Service
+public class InvitationService {
+    
+    @Autowired
+    private InvitationRepository invitationRepository;
+    
+    @Autowired
+    private TagRepository tagRepository;
+    
+    @Autowired
+    private UserRepository userRepository;
+    
+    @Transactional
+    public InvitationResponse createInvitation(CreateInvitationRequest request, String userEmail) {
+        User creator = userRepository.findByEmail(userEmail)
+            .orElseThrow(() -> new RuntimeException("User not found"));
+        
+        Tag tag = tagRepository.findById(request.getTagId())
+            .orElseThrow(() -> new RuntimeException("Tag not found"));
+        
+        Invitation invitation = new Invitation(
+            request.getTitle(),
+            request.getDescription(),
+            request.getMaxParticipants(),
+            tag,
+            creator
+        );
+        
+        invitation.setDateTime(request.getDateTime());
+        invitation.setLocation(request.getLocation());
+        
+        Invitation saved = invitationRepository.save(invitation);
+        return mapToResponse(saved);
+    }
+    
+    public Optional<InvitationResponse> getInvitationById(Long id) {
+        return invitationRepository.findById(id)
+            .map(this::mapToResponse);
+    }
+    
+    private InvitationResponse mapToResponse(Invitation invitation) {
+        InvitationResponse response = new InvitationResponse();
+        response.setId(invitation.getId());
+        response.setTitle(invitation.getTitle());
+        response.setDescription(invitation.getDescription());
+        response.setDateTime(invitation.getDateTime());
+        response.setLocation(invitation.getLocation());
+        response.setMaxParticipants(invitation.getMaxParticipants());
+        response.setCurrentAttendees(invitation.getCurrentAttendees());
+        response.setCreatedAt(invitation.getCreatedAt());
+        
+        InvitationResponse.TagDto tagDto = new InvitationResponse.TagDto();
+        tagDto.setId(invitation.getTag().getId());
+        tagDto.setName(invitation.getTag().getName());
+        tagDto.setColorHex(invitation.getTag().getColorHex());
+        tagDto.setIconName(invitation.getTag().getIconName());
+        response.setTag(tagDto);
+        
+        InvitationResponse.UserDto userDto = new InvitationResponse.UserDto();
+        userDto.setId(invitation.getCreator().getId());
+        userDto.setDisplayName(invitation.getCreator().getDisplayName());
+        userDto.setAvatar(invitation.getCreator().getAvatar());
+        response.setCreator(userDto);
+        
+        return response;
+    }
+}