feat: add user selection flow

backend/src/main/java/se/rubble/hemhub/api/ApiError.java

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+package se.rubble.hemhub.api;
+
+public record ApiError(String code, String message) {
+}
+

backend/src/main/java/se/rubble/hemhub/api/ApiExceptionHandler.java

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+package se.rubble.hemhub.api;
+
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.ExceptionHandler;
+import org.springframework.web.bind.annotation.RestControllerAdvice;
+
+import se.rubble.hemhub.user.InvalidUserNameException;
+import se.rubble.hemhub.user.UserNameAlreadyExistsException;
+
+@RestControllerAdvice
+public class ApiExceptionHandler {
+
+    @ExceptionHandler(InvalidUserNameException.class)
+    public ResponseEntity<ApiError> handleInvalidUserName() {
+        return ResponseEntity.badRequest()
+                .body(new ApiError(
+                        "INVALID_USER_NAME",
+                        "Namnet måste innehålla mellan 1 och 50 tecken."));
+    }
+
+    @ExceptionHandler(UserNameAlreadyExistsException.class)
+    public ResponseEntity<ApiError> handleDuplicateUserName() {
+        return ResponseEntity.status(HttpStatus.CONFLICT)
+                .body(new ApiError(
+                        "USER_NAME_ALREADY_EXISTS",
+                        "En användare med det namnet finns redan."));
+    }
+}
+

backend/src/main/java/se/rubble/hemhub/user/CreateUserRequest.java

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+package se.rubble.hemhub.user;
+
+public record CreateUserRequest(String name) {
+}
+

backend/src/main/java/se/rubble/hemhub/user/InvalidUserNameException.java

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+package se.rubble.hemhub.user;
+
+public class InvalidUserNameException extends RuntimeException {
+}
+

backend/src/main/java/se/rubble/hemhub/user/User.java

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+package se.rubble.hemhub.user;
+
+import java.time.Instant;
+import java.util.UUID;
+
+import jakarta.persistence.Column;
+import jakarta.persistence.Entity;
+import jakarta.persistence.Id;
+import jakarta.persistence.Table;
+
+@Entity
+@Table(name = "app_user")
+class User {
+
+    @Id
+    private UUID id;
+
+    @Column(nullable = false, length = 50)
+    private String name;
+
+    @Column(name = "normalized_name", nullable = false, length = 150, unique = true)
+    private String normalizedName;
+
+    @Column(name = "created_at", nullable = false)
+    private Instant createdAt;
+
+    protected User() {
+    }
+
+    User(UUID id, String name, String normalizedName, Instant createdAt) {
+        this.id = id;
+        this.name = name;
+        this.normalizedName = normalizedName;
+        this.createdAt = createdAt;
+    }
+
+    UUID getId() {
+        return id;
+    }
+
+    String getName() {
+        return name;
+    }
+
+    Instant getCreatedAt() {
+        return createdAt;
+    }
+}

backend/src/main/java/se/rubble/hemhub/user/UserController.java

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+package se.rubble.hemhub.user;
+
+import java.util.List;
+
+import org.springframework.http.HttpStatus;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.RequestBody;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.ResponseStatus;
+import org.springframework.web.bind.annotation.RestController;
+
+@RestController
+@RequestMapping("/api/users")
+public class UserController {
+
+    private final UserService userService;
+
+    UserController(UserService userService) {
+        this.userService = userService;
+    }
+
+    @GetMapping
+    public List<UserResponse> findAll() {
+        return userService.findAll();
+    }
+
+    @PostMapping
+    @ResponseStatus(HttpStatus.CREATED)
+    public UserResponse create(@RequestBody(required = false) CreateUserRequest request) {
+        return userService.create(request == null ? null : request.name());
+    }
+}
+

backend/src/main/java/se/rubble/hemhub/user/UserNameAlreadyExistsException.java

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+package se.rubble.hemhub.user;
+
+public class UserNameAlreadyExistsException extends RuntimeException {
+}
+

backend/src/main/java/se/rubble/hemhub/user/UserRepository.java

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+package se.rubble.hemhub.user;
+
+import java.util.UUID;
+
+import org.springframework.data.jpa.repository.JpaRepository;
+
+interface UserRepository extends JpaRepository<User, UUID> {
+
+    boolean existsByNormalizedName(String normalizedName);
+}
+

backend/src/main/java/se/rubble/hemhub/user/UserResponse.java

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+package se.rubble.hemhub.user;
+
+import java.time.Instant;
+import java.util.UUID;
+
+public record UserResponse(UUID id, String name, Instant createdAt) {
+
+    static UserResponse from(User user) {
+        return new UserResponse(user.getId(), user.getName(), user.getCreatedAt());
+    }
+}
+

backend/src/main/java/se/rubble/hemhub/user/UserService.java

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+package se.rubble.hemhub.user;
+
+import java.time.Clock;
+import java.time.Instant;
+import java.util.Comparator;
+import java.util.List;
+import java.util.Locale;
+import java.util.UUID;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.dao.DataIntegrityViolationException;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+@Service
+class UserService {
+
+    private static final Comparator<User> BY_DISPLAY_NAME =
+            Comparator.comparing(User::getName, String.CASE_INSENSITIVE_ORDER)
+                    .thenComparing(User::getName)
+                    .thenComparing(User::getId);
+
+    private final UserRepository userRepository;
+    private final Clock clock;
+
+    @Autowired
+    UserService(UserRepository userRepository) {
+        this(userRepository, Clock.systemUTC());
+    }
+
+    UserService(UserRepository userRepository, Clock clock) {
+        this.userRepository = userRepository;
+        this.clock = clock;
+    }
+
+    @Transactional(readOnly = true)
+    List<UserResponse> findAll() {
+        return userRepository.findAll().stream()
+                .sorted(BY_DISPLAY_NAME)
+                .map(UserResponse::from)
+                .toList();
+    }
+
+    @Transactional
+    UserResponse create(String requestedName) {
+        String name = requestedName == null ? "" : requestedName.trim();
+
+        if (name.isEmpty() || name.codePointCount(0, name.length()) > 50) {
+            throw new InvalidUserNameException();
+        }
+
+        String normalizedName = name.toLowerCase(Locale.ROOT);
+
+        if (userRepository.existsByNormalizedName(normalizedName)) {
+            throw new UserNameAlreadyExistsException();
+        }
+
+        User user = new User(UUID.randomUUID(), name, normalizedName, Instant.now(clock));
+
+        try {
+            return UserResponse.from(userRepository.saveAndFlush(user));
+        } catch (DataIntegrityViolationException exception) {
+            throw new UserNameAlreadyExistsException();
+        }
+    }
+}