feat(caldav): app-specific passwords so MFA accounts can use CalDAV

CalDAV clients send only user+password over Basic Auth and can't provide a TOTP
code, so account passwords would bypass 2FA. Add revocable app passwords:

- models: AppPassword table (bcrypt hash, label, last_used); auto-created via
  create_all
- profile_router: GET/POST/DELETE /profile/app-passwords (plaintext shown once)
- dav_router: Basic Auth accepts any app password; the account password is
  accepted only when 2FA is disabled
- frontend: "App-Passwörter (CalDAV)" section in the profile modal (create/show-
  once/copy/revoke) + i18n (de/en); login hint now says app password

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/models.py

@@ -114,6 +114,26 @@ class UserSettings(Base):
     user = relationship("User", back_populates="settings")
 
 
+class AppPassword(Base):
+    """Per-device app-specific password for CalDAV (Basic Auth).
+
+    Keeps MFA intact: accounts with 2FA can't use their normal password over
+    CalDAV (clients can't send a TOTP code), so they authenticate with one of
+    these revocable app passwords instead. Only the bcrypt hash is stored.
+    """
+
+    __tablename__ = "app_passwords"
+
+    id = Column(Integer, primary_key=True, index=True)
+    user_id = Column(Integer, ForeignKey("users.id"), nullable=False)
+    label = Column(String(100), nullable=False)
+    password_hash = Column(String(255), nullable=False)
+    created_at = Column(String(50), nullable=True)
+    last_used_at = Column(String(50), nullable=True)
+
+    user = relationship("User")
+
+
 class LocalCalendar(Base):
     __tablename__ = "local_calendars"