Hlavicky X-MS365-* jako obycejne hodnoty, GET /users/{user_id}/calendar

- credentials: sifrovani AES-GCM/HKDF odstraneno, tri hlavicky se berou
  tak, jak jsou; bez hlavicek se pouziji hodnoty z prostredi
- zrusena promenna MS365_CREDENTIAL_ENCODING_SECRET a zavislost cryptography
- novy endpoint GET /users/{user_id}/calendar (objekt kalendare schranky,
  id a name) pro read-only overeni pristupu pres e-mail schranky
- C# ukazka posila hodnoty primo, CredentialEncoder smazan
- README: sekce o hlavickach vcetne PowerShell ukazky, zaznam zmen
- .gitignore: bin/ a obj/, zaverzovane obj/ soubory ukazky odstraneny z gitu

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

app/services.py

@@ -91,6 +91,10 @@ class CalendarService:
     def __init__(self, graph: MicrosoftGraphClient) -> None:
         self._graph = graph
 
+    async def get_calendar(self, user_id: str) -> Any:
+        """Default calendar of the mailbox (id, name, owner). Cheapest read-only access check."""
+        return await self._graph.request("GET", f"/users/{graph_segment(user_id)}/calendar")
+
     async def list_events(self, user_id: str, top: int = 25) -> Any:
         params = {
             "$top": top,