feat: track imported accounts per upstream key across platforms

Add upstream_key_account_links table mapping generated keys to remote
accounts per website/platform, surface imported_accounts on key
responses, and update website sync/routers to manage the links.

backend/app/models/website.py

@@ -42,6 +42,7 @@ class WebsiteGroupBinding(Base):
     percent: Mapped[str] = mapped_column(String(32), default="0")
     algorithm: Mapped[str] = mapped_column(String(64), default="priority_weighted_plus_percent")
     enabled: Mapped[bool] = mapped_column(Boolean, default=True)
+    platform: Mapped[Optional[str]] = mapped_column(String(64), nullable=True)
     created_at: Mapped[datetime] = mapped_column(DateTime, default=lambda: datetime.now(timezone.utc))
     updated_at: Mapped[datetime] = mapped_column(
         DateTime, default=lambda: datetime.now(timezone.utc), onupdate=lambda: datetime.now(timezone.utc)