feat: add authentication service and endpoints

src/main/java/com/slotline/user/AuthController.java

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+package com.slotline.user;
+
+import com.slotline.user.dto.LoginRequest;
+import com.slotline.user.dto.RegisterRequest;
+import com.slotline.user.dto.TokenResponse;
+import com.slotline.user.dto.UserResponse;
+import io.swagger.v3.oas.annotations.Operation;
+import io.swagger.v3.oas.annotations.security.SecurityRequirements;
+import io.swagger.v3.oas.annotations.tags.Tag;
+import jakarta.validation.Valid;
+import org.springframework.http.HttpStatus;
+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/auth")
+@Tag(name = "Authentication")
+@SecurityRequirements
+public class AuthController {
+
+    private final AuthService authService;
+
+    public AuthController(AuthService authService) {
+        this.authService = authService;
+    }
+
+    @PostMapping("/register")
+    @ResponseStatus(HttpStatus.CREATED)
+    @Operation(summary = "Register a new USER account")
+    public UserResponse register(@Valid @RequestBody RegisterRequest request) {
+        return authService.register(request);
+    }
+
+    @PostMapping("/login")
+    @Operation(summary = "Exchange credentials for a JWT bearer token")
+    public TokenResponse login(@Valid @RequestBody LoginRequest request) {
+        return authService.login(request);
+    }
+}

src/main/java/com/slotline/user/AuthService.java

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+package com.slotline.user;
+
+import com.slotline.common.InvalidCredentialsException;
+import com.slotline.common.UsernameTakenException;
+import com.slotline.config.SlotlineProperties;
+import com.slotline.organization.Organization;
+import com.slotline.organization.OrganizationService;
+import com.slotline.security.JwtService;
+import com.slotline.user.dto.LoginRequest;
+import com.slotline.user.dto.RegisterRequest;
+import com.slotline.user.dto.TokenResponse;
+import com.slotline.user.dto.UserResponse;
+import org.springframework.security.crypto.password.PasswordEncoder;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+import java.time.Instant;
+
+@Service
+public class AuthService {
+
+    private final UserRepository users;
+    private final OrganizationService organizations;
+    private final PasswordEncoder passwordEncoder;
+    private final JwtService jwtService;
+    private final SlotlineProperties properties;
+
+    public AuthService(UserRepository users, OrganizationService organizations, PasswordEncoder passwordEncoder,
+                       JwtService jwtService, SlotlineProperties properties) {
+        this.users = users;
+        this.organizations = organizations;
+        this.passwordEncoder = passwordEncoder;
+        this.jwtService = jwtService;
+        this.properties = properties;
+    }
+
+    @Transactional
+    public UserResponse register(RegisterRequest request) {
+        if (users.existsByUsername(request.username())) {
+            throw new UsernameTakenException(request.username());
+        }
+        Organization organization = resolveOrganization(request.organizationSlug());
+        User user = new User(organization.getId(), request.username(), passwordEncoder.encode(request.password()),
+                Role.USER, Instant.now());
+        users.save(user);
+        return new UserResponse(user.getId(), user.getUsername(), user.getRole(), user.getOrganizationId());
+    }
+
+    @Transactional(readOnly = true)
+    public TokenResponse login(LoginRequest request) {
+        User user = users.findByUsername(request.username())
+                .filter(candidate -> passwordEncoder.matches(request.password(), candidate.getPasswordHash()))
+                .orElseThrow(InvalidCredentialsException::new);
+        String token = jwtService.issue(user.getId(), user.getUsername(), user.getRole(), user.getOrganizationId());
+        return new TokenResponse(token, "Bearer", properties.getJwt().getExpiration().toSeconds());
+    }
+
+    private Organization resolveOrganization(String slug) {
+        String target = (slug == null || slug.isBlank()) ? OrganizationService.defaultSlug : slug;
+        return organizations.requireBySlug(target);
+    }
+}

src/main/java/com/slotline/user/dto/LoginRequest.java

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+package com.slotline.user.dto;
+
+import jakarta.validation.constraints.NotBlank;
+
+public record LoginRequest(
+        @NotBlank String username,
+        @NotBlank String password) {
+}

src/main/java/com/slotline/user/dto/RegisterRequest.java

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+package com.slotline.user.dto;
+
+import jakarta.validation.constraints.NotBlank;
+import jakarta.validation.constraints.Size;
+
+public record RegisterRequest(
+        @NotBlank @Size(min = 3, max = 64) String username,
+        @NotBlank @Size(min = 8, max = 72) String password,
+        @Size(max = 64) String organizationSlug) {
+
+    public RegisterRequest(String username, String password) {
+        this(username, password, null);
+    }
+}

src/main/java/com/slotline/user/dto/TokenResponse.java

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+package com.slotline.user.dto;
+
+public record TokenResponse(String token, String tokenType, long expiresInSeconds) {
+}

src/main/java/com/slotline/user/dto/UserResponse.java

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+package com.slotline.user.dto;
+
+import com.slotline.user.Role;
+
+public record UserResponse(Long id, String username, Role role, Long organizationId) {
+}