Add Jin10 QR connection with cancellable login tasks

Store account credentials encrypted and bind QR tasks to administrator sessions. Report calendar access as awaiting validation after login.

Prevent late polls and status responses from overwriting cancelled or newer operations, and clean up QR tasks when settings unmount. Cover authentication, cancellation races, and stale frontend responses.

backend/src/alphapulse/models/jin10.py

@@ -0,0 +1,21 @@
+"""Encrypted connection and short-lived, session-owned QR login state."""
+from datetime import datetime
+
+from sqlalchemy import DateTime, LargeBinary, String, Text
+from sqlalchemy.orm import Mapped, mapped_column
+
+from alphapulse.core.db import Base
+
+
+class Jin10Connection(Base):
+    __tablename__ = "jin10_connection"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    token_encrypted: Mapped[bytes | None] = mapped_column(LargeBinary)
+    connected_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    task_id: Mapped[str | None] = mapped_column(String(64))
+    owner: Mapped[str | None] = mapped_column(String(64))
+    qr_encrypted: Mapped[bytes | None] = mapped_column(LargeBinary)
+    expires_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    next_poll_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    error: Mapped[str | None] = mapped_column(Text)