fix: 登录有效期由 8 小时改为 30 天 (#42)

- 有效期提取为具名常量 SessionLifetime（绝对有效期，不滑动续期），学习端与管理端共用
  同一登录接口，一处生效；数据库结构与接口字段不变，旧会话按各自 expires_at 自然过渡。
- 过期断言改为按常量计算（临期仍可用、超过后被拒），新增测试锁定 30 天常量、
  登录响应与库内 expires_at 一致、库内只存 SHA-256 摘要。
- 撤销规则不变：退出撤销当前会话，改密码/停用/重置撤销该账号全部会话。
- Wiki 已在线上更新（会话规则数字、验证说明、Home/Project-Profile 记录）；镜像导出留到
  验收时执行，避免把尚未合入的 #40 段落带进本分支。

server/app/lexgo/integration_test.go

@@ -163,6 +163,55 @@ func TestMySQLLoginUsesStoredPasswordWithoutChangingCreationPolicy(t *testing.T)
 	}
 }
 
+// TestMySQLSessionLifetimeIsThirtyDays pins the login lifetime that both clients share. It checks
+// the API response, the stored row and the privacy rule (only a digest is persisted) so changing the
+// value without updating this test is impossible.
+func TestMySQLSessionLifetimeIsThirtyDays(t *testing.T) {
+	if SessionLifetime != 30*24*time.Hour {
+		t.Fatalf("session lifetime %s, want 30 days", SessionLifetime)
+	}
+	db := testDB(t)
+	u := admin.SysUser{Username: randomName("sesslife"), Password: fixturePassword, RoleId: 2, Status: "2"}
+	if err := db.Create(&u).Error; err != nil {
+		t.Fatal(err)
+	}
+	r := Router(db, time.Now)
+	before := time.Now()
+	code, data := callAPI(t, r, "POST", "/api/v1/login", "", map[string]string{"username": u.Username, "password": fixturePassword})
+	if code != 200 {
+		t.Fatalf("login status %d", code)
+	}
+	var login struct {
+		Token     string    `json:"token"`
+		ExpiresAt time.Time `json:"expiresAt"`
+	}
+	if err := json.Unmarshal(data, &login); err != nil {
+		t.Fatal(err)
+	}
+	granted := login.ExpiresAt.Sub(before)
+	if granted < SessionLifetime-time.Minute || granted > SessionLifetime+time.Minute {
+		t.Fatalf("granted %s, want about %s", granted, SessionLifetime)
+	}
+	var stored Session
+	if err := db.Where("token_hash = ?", digest(login.Token)).First(&stored).Error; err != nil {
+		t.Fatal("session row missing")
+	}
+	if stored.ExpiresAt.Sub(stored.ExpiresAt.Truncate(time.Millisecond)) != 0 {
+		t.Fatal("stored expiry lost precision")
+	}
+	if delta := login.ExpiresAt.Sub(stored.ExpiresAt); delta > time.Millisecond || delta < -time.Millisecond {
+		t.Fatalf("stored expiry %s differs from the response by %s", stored.ExpiresAt, delta)
+	}
+	var raw int64
+	db.Table("lexgo_sessions").Where("token_hash = ?", login.Token).Count(&raw)
+	if raw != 0 {
+		t.Fatal("raw token persisted")
+	}
+	if code, _ = callAPI(t, r, "GET", "/api/v1/me", login.Token, nil); code != 200 {
+		t.Fatalf("fresh session rejected: %d", code)
+	}
+}
+
 func TestMySQLAccountIsolationAndRevocation(t *testing.T) {
 	db := testDB(t)
 	if err := Migrate(db); err != nil {
@@ -281,7 +330,13 @@ func TestMySQLAccountIsolationAndRevocation(t *testing.T) {
 		t.Fatal("old password accepted")
 	}
 	loginToken(t, r, users[0], "replacement-fixture-pass")
-	clock = clock.Add(9 * time.Hour)
+	// The lifetime is absolute, so a session is still valid just before it and gone right after;
+	// asserting against SessionLifetime keeps this test honest when the value changes.
+	clock = clock.Add(SessionLifetime - time.Hour)
+	if code, _ = callAPI(t, r, "GET", "/api/v1/me", tokenB, nil); code != 200 {
+		t.Fatal("session must still be valid before its lifetime ends", code)
+	}
+	clock = clock.Add(2 * time.Hour)
 	code, _ = callAPI(t, r, "GET", "/api/v1/me", tokenB, nil)
 	if code != 401 {
 		t.Fatal("expired session valid")

server/app/lexgo/service.go

@@ -38,6 +38,11 @@ type Space struct {
 
 func (Space) TableName() string { return "lexgo_spaces" }
 
+// SessionLifetime is how long a login lasts. It is absolute, not sliding: using the app does not
+// extend it, so a stolen token cannot be kept alive forever. Both the learner and the admin client
+// authenticate through the same login endpoint, so one value covers them.
+const SessionLifetime = 30 * 24 * time.Hour
+
 type Session struct {
 	TokenHash string `gorm:"primaryKey"`
 	OwnerID   int
@@ -162,7 +167,7 @@ func login(db *gorm.DB, now time.Time, name, password string) (LoginResult, erro
 			return err
 		}
 		token := hex.EncodeToString(b)
-		expiry := now.Add(8 * time.Hour)
+		expiry := now.Add(SessionLifetime)
 		if err = tx.Where("owner_id = ? AND expires_at <= ?", u.UserId, now).Delete(&Session{}).Error; err != nil {
 			return err
 		}