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/config.py

@@ -10,7 +10,6 @@ class Settings:
     tenant_id: str = os.getenv("MS365_TENANT_ID", "")
     client_id: str = os.getenv("MS365_CLIENT_ID", "")
     client_secret: str = os.getenv("MS365_CLIENT_SECRET", "")
-    credential_encoding_secret: str = os.getenv("MS365_CREDENTIAL_ENCODING_SECRET", "")
     graph_base_url: str = os.getenv("MS365_GRAPH_BASE_URL", "https://graph.microsoft.com/v1.0")
     graph_scope: str = os.getenv("MS365_GRAPH_SCOPE", "https://graph.microsoft.com/.default")
     request_timeout_seconds: float = float(os.getenv("MS365_REQUEST_TIMEOUT_SECONDS", "30"))