Google Calendar OAuth2 Integration + CalDAV-Kalender ausblenden statt löschen

- Google OAuth2 Flow: Admin konfiguriert Client-ID/Secret, User verbindet per Klick
- Google Calendar API v3: Events lesen, erstellen, bearbeiten, löschen
- GoogleAccount Model + google_router mit Token-Refresh
- Google-Events in Event-Pipeline integriert
- Frontend: Google Kalender in Sidebar, Dropdown, Event-CRUD-Routing
- CalDAV-Kalender: Ausblenden statt ganzes Konto löschen, Einblenden in Einstellungen
- Ausgeblendete Kalender Sektion in Einstellungen

backend/models.py

@@ -27,6 +27,9 @@ class User(Base):
     ical_subscriptions = relationship(
         "ICalSubscription", back_populates="user", cascade="all, delete-orphan"
     )
+    google_accounts = relationship(
+        "GoogleAccount", back_populates="user", cascade="all, delete-orphan"
+    )
 
 
 class CalDAVAccount(Base):
@@ -138,3 +141,16 @@ class ICalOverride(Base):
     color = Column(String(7), nullable=True)
 
     subscription = relationship("ICalSubscription", back_populates="overrides")
+
+
+class GoogleAccount(Base):
+    __tablename__ = "google_accounts"
+
+    id = Column(Integer, primary_key=True, index=True)
+    user_id = Column(Integer, ForeignKey("users.id"), nullable=False)
+    email = Column(String(255), nullable=False)
+    access_token = Column(Text, nullable=False)
+    refresh_token = Column(Text, nullable=False)
+    token_expiry = Column(DateTime, nullable=True)
+
+    user = relationship("User", back_populates="google_accounts")