Reconcile Trajectories

Mark server-side trajectories as archived when the local client no longer has them. Trajectories are append-only history — the file on disk + summary + DB row are NEVER deleted, just flagged with archived_at. The default list endpoint hides archived; pass ?include_archived=1 to surface them again.

cloud_sync calls this once per sweep with the full list of session_ids it sees on disk; anything the server holds for the calling user that isn't in the list (and isn't already archived) gets archived. Re-syncing a session that was previously archived re-activates it (clears archived_at) — useful when a user restores a trajectory locally.

Body: {"session_ids": ["...", "..."]}. Scope = the caller's owned trajectories only.