feat(e2e): shared initData / Login Widget payload builder for E2E tests

- Add TelegramAuthPayloadBuilder in GmRelay.Shared for C# tests.
- Refactor TelegramAuthServiceTests to use the shared builder.
- Add Python equivalent (telegram_init_data.py) for E2E runner.
- Add self-contained Python tests and E2E README.

Closes #144

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/e2e/README.md

@@ -0,0 +1,71 @@
+# GmRelay E2E Tests
+
+This directory contains end-to-end tests that run **locally** against real Telegram infrastructure and the GmRelay Web dashboard. They are intentionally **not part of CI** because they require:
+
+- a real Telegram test user account;
+- `api_id` / `api_hash` from https://my.telegram.org;
+- a pre-authenticated MTProto session;
+- a running PostgreSQL, GmRelay.Bot, and GmRelay.Web.
+
+## Structure
+
+```text
+tests/e2e/
+├── README.md                        # this file
+├── helpers/
+│   ├── telegram_init_data.py        # generate valid Telegram initData / Login Widget payloads
+│   └── test_telegram_init_data.py   # unit tests for the helper
+└── ... (runner and scenarios will land here)
+```
+
+## `telegram_init_data.py`
+
+A small Python helper that produces Telegram Mini App `initData` and Login Widget payloads with valid HMAC-SHA256 signatures. It mirrors `GmRelay.Shared.Telegram.TelegramAuthPayloadBuilder`.
+
+Use it to open the Blazor/Mini App dashboard in Playwright without logging into Telegram:
+
+```python
+from helpers.telegram_init_data import build_mini_app_init_data
+
+init = build_mini_app_init_data(
+    bot_token="YOUR_BOT_TOKEN",
+    telegram_id=424242,
+    first_name="Test",
+    username="tester")
+
+await page.goto(f"https://localhost:8080/#tgWebAppData={init.init_data_raw}")
+```
+
+## C# equivalent
+
+For tests inside the solution, use `GmRelay.Shared.Telegram.TelegramAuthPayloadBuilder`:
+
+```csharp
+var init = TelegramAuthPayloadBuilder.BuildMiniAppInitData(
+    botToken: "YOUR_BOT_TOKEN",
+    telegramId: 424242L,
+    firstName: "Test",
+    username: "tester");
+
+// init.InitDataRaw is a valid initData string.
+```
+
+## Running the helper tests
+
+```bash
+cd tests/e2e/helpers
+python -m pytest test_telegram_init_data.py -v
+```
+
+## Roadmap
+
+See the Gitea milestone **[Этап — E2E-тестирование Telegram + Web](https://git.codeanddice.ru/Toutsu/GmRelayBot/milestone/13)** and its issues:
+
+- [#144](https://git.codeanddice.ru/Toutsu/GmRelayBot/issues/144) initData helper ✅ (this directory)
+- [#145](https://git.codeanddice.ru/Toutsu/GmRelayBot/issues/145) Playwright dashboard tests
+- [#146](https://git.codeanddice.ru/Toutsu/GmRelayBot/issues/146) MTProto test user client
+- [#147](https://git.codeanddice.ru/Toutsu/GmRelayBot/issues/147) Group creation automation
+- [#148](https://git.codeanddice.ru/Toutsu/GmRelayBot/issues/148) `/newsession` scenario
+- [#149](https://git.codeanddice.ru/Toutsu/GmRelayBot/issues/149) join/leave/waitlist/reschedule scenarios
+- [#150](https://git.codeanddice.ru/Toutsu/GmRelayBot/issues/150) Web dashboard round-trip verification
+- [#151](https://git.codeanddice.ru/Toutsu/GmRelayBot/issues/151) Console runner + cleanup