feat(billing): add INTERNAL_TESTER_EMAILS allowlist for self-serve soft cutover

Phase O Task 46 needs internal validation of the full self-serve flow
against the prod backend before flipping SELF_SERVE_ENABLED public. This
adds the per-email allowlist that bypasses the global flag for specific
authenticated users.

- INTERNAL_TESTER_EMAILS: comma-separated list, parsed by a Pydantic
  field_validator into a normalized lowercase list. Settings.is_internal_tester
  and Settings.is_self_serve_active_for centralize the allowlist + global-flag
  check; both endpoints below call the latter.
- New get_current_user_optional dep — best-effort auth that returns None
  on missing/invalid token instead of 401. Used by /config/public so the
  same endpoint serves anonymous public callers and authenticated allowlist
  members.
- /config/public now accepts optional auth and returns self_serve_enabled=True
  for authenticated allowlist members even when the global flag is off.
  Anonymous callers always see the global flag.
- /auth/register replaces the SELF_SERVE_ENABLED check with the helper so a
  registering email on the allowlist can join without an invite code.
  Non-allowlist emails still 400 when self-serve is off.
- docker-compose.dev.yml passes SELF_SERVE_ENABLED + INTERNAL_TESTER_EMAILS
  through; backend/.env.example documents both.

Tests cover: allowlisted authenticated user sees true, non-allowlisted
authenticated user sees the global flag, anonymous calls ignore the
allowlist, allowlisted email registers without invite code, non-allowlisted
email still blocked.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/tests/test_config_public.py

@@ -49,6 +49,58 @@ class TestConfigPublic:
         assert response.status_code == 200
         assert response.json()["oauth_providers"] == ["microsoft"]
 
+    @pytest.mark.asyncio
+    async def test_get_config_public_returns_true_for_internal_tester(
+        self,
+        client: AsyncClient,
+        auth_headers: dict,
+        test_user: dict,
+        monkeypatch: pytest.MonkeyPatch,
+    ):
+        """Authenticated user whose email is on INTERNAL_TESTER_EMAILS sees
+        self_serve_enabled=True even when the global flag is off."""
+        monkeypatch.setattr(settings, "SELF_SERVE_ENABLED", False)
+        monkeypatch.setattr(settings, "GOOGLE_CLIENT_ID", None)
+        monkeypatch.setattr(settings, "MS_CLIENT_ID", None)
+        monkeypatch.setattr(settings, "INTERNAL_TESTER_EMAILS", [test_user["email"].lower()])
+
+        response = await client.get("/api/v1/config/public", headers=auth_headers)
+        assert response.status_code == 200
+        assert response.json()["self_serve_enabled"] is True
+
+    @pytest.mark.asyncio
+    async def test_get_config_public_returns_false_for_non_tester_when_global_off(
+        self,
+        client: AsyncClient,
+        auth_headers: dict,
+        monkeypatch: pytest.MonkeyPatch,
+    ):
+        """Authenticated user NOT on the allowlist sees the global flag —
+        prevents accidental opt-in via stale credentials or empty allowlist."""
+        monkeypatch.setattr(settings, "SELF_SERVE_ENABLED", False)
+        monkeypatch.setattr(settings, "GOOGLE_CLIENT_ID", None)
+        monkeypatch.setattr(settings, "MS_CLIENT_ID", None)
+        monkeypatch.setattr(settings, "INTERNAL_TESTER_EMAILS", ["someone-else@example.com"])
+
+        response = await client.get("/api/v1/config/public", headers=auth_headers)
+        assert response.status_code == 200
+        assert response.json()["self_serve_enabled"] is False
+
+    @pytest.mark.asyncio
+    async def test_get_config_public_anonymous_ignores_allowlist(
+        self, client: AsyncClient, monkeypatch: pytest.MonkeyPatch
+    ):
+        """Anonymous callers always see the global flag — the allowlist is
+        keyed on authenticated identity, not request content."""
+        monkeypatch.setattr(settings, "SELF_SERVE_ENABLED", False)
+        monkeypatch.setattr(settings, "GOOGLE_CLIENT_ID", None)
+        monkeypatch.setattr(settings, "MS_CLIENT_ID", None)
+        monkeypatch.setattr(settings, "INTERNAL_TESTER_EMAILS", ["anon-tester@example.com"])
+
+        response = await client.get("/api/v1/config/public")
+        assert response.status_code == 200
+        assert response.json()["self_serve_enabled"] is False
+
 
 class TestRegisterInviteCodeGate:
     """Regression + new-behavior tests for /auth/register vs SELF_SERVE_ENABLED."""
@@ -98,3 +150,55 @@ class TestRegisterInviteCodeGate:
         assert body["email"] == "self-serve@example.com"
         assert body["account_role"] == "owner"
         assert "account_id" in body
+
+    @pytest.mark.asyncio
+    async def test_register_invite_code_optional_for_internal_tester(
+        self, client: AsyncClient, monkeypatch: pytest.MonkeyPatch
+    ):
+        """SELF_SERVE_ENABLED is False but the registering email is on
+        INTERNAL_TESTER_EMAILS — registration should succeed without an
+        invite code, matching the per-email soft-cutover behavior."""
+        monkeypatch.setattr(settings, "REQUIRE_INVITE_CODE", True)
+        monkeypatch.setattr(settings, "SELF_SERVE_ENABLED", False)
+        monkeypatch.setattr(
+            settings, "INTERNAL_TESTER_EMAILS", ["tester@example.com"]
+        )
+
+        response = await client.post(
+            "/api/v1/auth/register",
+            json={
+                "email": "tester@example.com",
+                "password": "SecurePass123!",
+                "name": "Internal Tester",
+            },
+        )
+
+        assert response.status_code == 201, response.text
+        body = response.json()
+        assert body["email"] == "tester@example.com"
+        assert body["account_role"] == "owner"
+
+    @pytest.mark.asyncio
+    async def test_register_blocked_for_non_tester_when_self_serve_disabled(
+        self, client: AsyncClient, monkeypatch: pytest.MonkeyPatch
+    ):
+        """Registering with an email NOT on the allowlist still 400s when
+        self-serve is off and no invite code is provided. Prevents the
+        allowlist from leaking to public users."""
+        monkeypatch.setattr(settings, "REQUIRE_INVITE_CODE", True)
+        monkeypatch.setattr(settings, "SELF_SERVE_ENABLED", False)
+        monkeypatch.setattr(
+            settings, "INTERNAL_TESTER_EMAILS", ["other@example.com"]
+        )
+
+        response = await client.post(
+            "/api/v1/auth/register",
+            json={
+                "email": "outsider@example.com",
+                "password": "SecurePass123!",
+                "name": "Outsider",
+            },
+        )
+
+        assert response.status_code == 400
+        assert "invite code is required" in response.json()["detail"].lower()