扫码登录：cdrop 自签会话原语 + 薄账户层 + 受限访客 + 2FA step-up

身份仍源自 OAuth provider（user_id = OIDC sub），cdrop 在其上自维护一薄层：
自签会话令牌 + accounts 表，并新增扫码快速登录。实施蓝本见 AUTH.md。

后端 · 自签会话原语
- web_sessions 加 kind/scope/granted_by 列（bootstrap 幂等迁移）；既有 oidc
  会话行为不变，新增 self（滑动续期）/ guest（受限·不续）两类
- cdrop 自签 HS256 会话 access token，密钥派生自 SESSION_SECRET，与落盘 AES、
  shortcut HS256 三密钥域隔离；jwtauth.verifySelfToken 无状态校验、靠 typ 区分
- requireFullSession 守卫：guest 会话不得改账号 / 再批准设备 / 签长效 token
- /auth/refresh 按 kind 分流：self/guest 纯自签、不触 IdP

后端 · 扫码登录（internal/httpapi/qr.go）
- qr/start・status・request・approve・deny 五端点 + login_requests 表 + reaper
- 三密钥分离：QR 仅含批准信息，会话只投递给持私有 poll_secret 的原设备
  （偷拍 QR 者无 poll_secret 领不到会话、未登录批不了准）
- 会话在新设备侧领取（cookie 不经手机）、单次消费、短 TTL

后端 · 薄账户层与 2FA step-up
- accounts 表（键 sub，不含任何凭证）：exchange/refresh upsert match_key /
  显示名 / 头像 / roles，供显示与未来管理员开启迁移标记时跨源关联
- step-up（默认关）：开启后 qr/approve 要求新鲜 prompt=login 授权码，后端就地
  换 id_token、JWKS 验签 + auth_time 窗口 + sub 匹配，provider 2FA 于此往返强制

前端（web/）
- 显码页 /link/new + 批准页 /link + net/qr.ts，对齐 Theme B、复用 AuthShell
  与聚珍排版管线、零新全局样式
- step-up 再认证流：批准前跑 prompt=login PKCE，回调分叉（不消费一次性 code、
  独立 state key）后带 step_up_code/verifier 调 approve
- 三语 i18n qr.*；新增 qrcode 依赖

修复 · clipboard sweeper（早已提交的损坏）
- ClearExpiredClipboards 因 clipboard.sql 全角注释触发 sqlc 1.31 多字节偏移
  bug，生成 SQL 被截断为「UPDATE clipboard_state SET content =」，sweeper 运行
  期报「incomplete input」、过期剪贴板内容从未清除（短 TTL 暴露保护失效）
- 注释改纯 ASCII 并加 bug 警告，重生成得完整 SQL；prod 已验证 sweeper 由 ERROR
  转为正常清理（cleared count=1）

构建
- .dockerignore：排除本地 node_modules 等，避免宿主原生二进制污染镜像内 vite 构建
- Dockerfile.base：GOPROXY 改为可经 --build-arg 覆盖（默认仍官方代理，受限网络
  构建时传区域镜像即可，仓库不固化区域值）

文档
- 新增 AUTH.md（账户与登录实施蓝本）；README 特性；.env.example /
  compose.snippet 增配置项（QR / 自签会话 TTL / step-up / match_claim）

测试
- 自签 token 密钥域隔离、扫码端到端（领取 / 单次 / poll_secret 校验 / deny /
  step-up 门）、extractIdentity、web_sessions 升级迁移

internal/jwtauth/middleware_test.go

@@ -488,3 +488,80 @@ func TestSanitizeDeviceName(t *testing.T) {
 		}
 	}
 }
+
+// signSelfToken mints a cdrop self-signed session token the way httpapi.mintSessionToken
+// does: HS256 over DeriveSessionTokenKey, with typ + scope custom claims and no jti.
+func signSelfToken(t *testing.T, secret, sub, typ, scope string, exp time.Time) string {
+	t.Helper()
+	sig, err := jose.NewSigner(
+		jose.SigningKey{Algorithm: jose.HS256, Key: DeriveSessionTokenKey(secret)},
+		(&jose.SignerOptions{}).WithType("JWT"),
+	)
+	if err != nil {
+		t.Fatalf("signer: %v", err)
+	}
+	std := jwt.Claims{
+		Subject:  sub,
+		IssuedAt: jwt.NewNumericDate(time.Now()),
+		Expiry:   jwt.NewNumericDate(exp),
+	}
+	tok, err := jwt.Signed(sig).Claims(std).Claims(map[string]any{"typ": typ, "scope": scope}).Serialize()
+	if err != nil {
+		t.Fatalf("serialize: %v", err)
+	}
+	return tok
+}
+
+// A cdrop self-signed session token authenticates as a full or guest session;
+// guest is capability-limited (Guest() true). A wrong typ or an unknown scope is
+// rejected, and the SessionSecret-derived key is domain-isolated from the HS256
+// shortcut key so the two token families never cross-validate (AUTH.md §3.1).
+func TestSelfToken_ScopeAndKeyIsolation(t *testing.T) {
+	secret := "test-session-secret-at-least-32-bytes-long"
+	a := New(&config.Config{AuthMode: "prod", SessionSecret: secret}, &fakeDeviceUpserter{})
+	exp := time.Now().Add(time.Hour)
+
+	// Full session: authenticates, not scoped, not guest.
+	code, claims := serveBearer(a, signSelfToken(t, secret, "user-x", "session", "full", exp))
+	if code != http.StatusOK {
+		t.Fatalf("full session token: got %d, want 200", code)
+	}
+	if claims == nil || claims.UserID != "user-x" || claims.SessionScope != "full" || claims.Scoped() || claims.Guest() {
+		t.Fatalf("full session claims wrong: %+v", claims)
+	}
+
+	// Guest session: authenticates and is marked guest (capability-limited).
+	code, claims = serveBearer(a, signSelfToken(t, secret, "user-x", "session", "guest", exp))
+	if code != http.StatusOK || claims == nil || !claims.Guest() || claims.Scoped() {
+		t.Fatalf("guest session: code=%d claims=%+v", code, claims)
+	}
+
+	// Wrong typ (not "session") signed with the session key → rejected.
+	if c, _ := serveBearer(a, signSelfToken(t, secret, "user-x", "other", "full", exp)); c != http.StatusUnauthorized {
+		t.Errorf("wrong typ: got %d, want 401", c)
+	}
+	// Unknown scope → rejected (no privilege-by-typo).
+	if c, _ := serveBearer(a, signSelfToken(t, secret, "user-x", "session", "admin", exp)); c != http.StatusUnauthorized {
+		t.Errorf("invalid scope: got %d, want 401", c)
+	}
+
+	// Key-domain isolation: with BOTH secrets set, the two families stay disjoint —
+	// a session token never becomes scoped, a shortcut token never becomes a session.
+	hsSecret := "test-hs256-secret-at-least-32-bytes-long"
+	store := &fakeDeviceUpserter{tokens: map[string]db.ShortcutToken{
+		"jti-1": {Jti: "jti-1", UserID: "user-x", Scopes: "clipboard", ExpiresAt: exp.Unix()},
+	}}
+	ab := New(&config.Config{AuthMode: "prod", SessionSecret: secret, HS256Secret: hsSecret}, store)
+	if _, sc := serveBearer(ab, signSelfToken(t, secret, "user-x", "session", "guest", exp)); sc == nil || sc.Scoped() || !sc.Guest() {
+		t.Errorf("session token leaked into scoped path: %+v", sc)
+	}
+	if _, hc := serveBearer(ab, signHS256(t, hsSecret, "user-x", "jti-1", "clipboard", exp)); hc == nil || !hc.Scoped() || hc.SessionScope != "" {
+		t.Errorf("shortcut token leaked into session path: %+v", hc)
+	}
+
+	// No SessionSecret on the server → self tokens are unverifiable (key nil).
+	noSecret := New(&config.Config{AuthMode: "prod", HS256Secret: hsSecret}, &fakeDeviceUpserter{})
+	if c, _ := serveBearer(noSecret, signSelfToken(t, secret, "user-x", "session", "full", exp)); c != http.StatusUnauthorized {
+		t.Errorf("self token without server SessionSecret must be rejected, got %d", c)
+	}
+}