chore: update project documentation and configuration files

- Added AGENTS.md for AI agent documentation and maintainers, outlining project structure and conventions.
- Updated CONTRIBUTING.md to specify that all project documentation must be in English, including README and docstrings.
- Enhanced README.md to reference documentation guidelines and the new AGENTS.md file.
- Cleaned up .gitignore by removing unnecessary entries for cursor-related files.
- Introduced new .cursor rules for backend, frontend, project architecture, and testing to standardize development practices.

.cursor/rules/testing.mdc

@@ -0,0 +1,103 @@
+---
+description: Rules for writing and running tests
+globs:
+  - tests/**
+  - webapp/js/**/*.test.js
+---
+
+# Testing
+
+## Python tests (pytest)
+
+### Configuration
+
+- Config in `pyproject.toml` under `[tool.pytest.ini_options]`.
+- `asyncio_mode = "auto"` — async test functions are detected automatically, no decorator needed.
+- Coverage: `--cov=duty_teller --cov-fail-under=80`.
+- Run: `pytest tests/ -v` from project root.
+
+### File layout
+
+```
+tests/
+├── conftest.py       # Shared fixtures (in-memory DB, sessions, bot app)
+├── helpers.py        # Test helper functions
+└── test_*.py         # Test modules
+```
+
+### Key fixtures (conftest.py)
+
+- `conftest.py` sets `BOT_TOKEN=test-token-for-pytest` if the env var is missing,
+  so tests run without a real token.
+- Database fixtures use in-memory SQLite for isolation.
+
+### Writing Python tests
+
+- File naming: `tests/test_<module>.py` (e.g. `tests/test_import_service.py`).
+- Function naming: `test_<what>_<scenario>` or `test_<what>_<expected_result>`.
+- Use `session_scope` with the test database URL for DB tests.
+- Async tests: just use `async def test_...` — `asyncio_mode = "auto"` handles it.
+- Mock external dependencies (Telegram API, HTTP calls) with `unittest.mock` or `pytest-mock`.
+
+### Example structure
+
+```python
+import pytest
+from duty_teller.db.session import session_scope
+
+def test_get_or_create_user_creates_new(test_db_url):
+    with session_scope(test_db_url) as session:
+        user = get_or_create_user(session, telegram_user_id=123, full_name="Test")
+        assert user.telegram_user_id == 123
+```
+
+## JavaScript tests (Vitest)
+
+### Configuration
+
+- Config: `webapp/vitest.config.js`.
+- Environment: `happy-dom` (lightweight DOM implementation).
+- Test files: `webapp/js/**/*.test.js`.
+- Run: `npm run test` (from `webapp/`).
+
+### DOM setup
+
+Modules that import from `dom.js` expect DOM elements to exist at import time.
+Use `beforeAll` to set up the required HTML structure before the first import:
+
+```js
+import { beforeAll, describe, it, expect } from "vitest";
+
+beforeAll(() => {
+  document.body.innerHTML = `
+    <div id="calendar"></div>
+    <h2 id="monthTitle"></h2>
+    <button id="prevBtn"></button>
+    <button id="nextBtn"></button>
+    <div id="dutyList"></div>
+    <div id="dayDetail"></div>
+    <div id="accessDenied" class="hidden"></div>
+    <div id="errorBanner" class="hidden"></div>
+  `;
+});
+```
+
+### Writing JS tests
+
+- File naming: `webapp/js/<module>.test.js` (co-located with the source module).
+- Test pure functions first (`dateUtils`, `i18n`, `utils`); mock DOM for render tests.
+- Use `describe` blocks to group by function, `it` blocks for individual cases.
+
+### Example structure
+
+```js
+import { describe, it, expect } from "vitest";
+import { localDateString } from "./dateUtils.js";
+
+describe("localDateString", () => {
+  it("formats date as YYYY-MM-DD", () => {
+    const d = new Date(2025, 0, 15);
+    expect(localDateString(d)).toBe("2025-01-15");
+  });
+});
+```