Eject from Linear: cutover

Tracking issue for turning Linear off once the blockers land. Steps: (1) flip tesks.tasks.provider to PostgresTaskSystem + dual-write=false; (2) final POST /admin/backfill to capture Linear-only edits; (3) UI hybrid branch (team.linear_id) just takes the native path — no UI change needed; (4) remove the Linear API key. Blocked by: PostgresTaskSystem, per-team number sequence, identity convergence.