Add test client and automated integration test suite

- 'test' OAuth client maps to stefan's data via USER_ALIASES
- 38 tests covering OAuth (metadata, client_credentials, PKCE, invalid
  secret, no auth), Mail (accounts, folders, search), Calendar
  (calendars, tasks, events, search), Contacts (search, empty),
  Files (list, info), Notes (notebooks)
- Daily systemd timer (05:00) with NTFY notification on failure
- Shared token store (.active_tokens.json) for cross-process auth

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

common.py

@@ -23,7 +23,7 @@ from starlette.middleware.base import BaseHTTPMiddleware
 BASE_DIR = os.path.dirname(__file__)
 TOKENS_FILE = os.path.join(BASE_DIR, "tokens.json")
 CONFIG_FILE = os.path.join(BASE_DIR, "config.json")
-VALID_USERS = ["stefan", "kati"]
+VALID_USERS = ["stefan", "kati", "test"]
 
 _config_cache = None
 
@@ -61,8 +61,11 @@ def _load_tokens():
     return _tokens_cache
 
 
+USER_ALIASES = {"test": "stefan"}
+
 def get_current_user() -> str | None:
-    return _current_user.get()
+    user = _current_user.get()
+    return USER_ALIASES.get(user, user)
 
 
 def get_user_key(username: str) -> str: