perf(gpu-queue): partial indexes + two-phase lease so leasing stays O(batch)

The throughput bottleneck was curator-side, not the network. lease() claimed the
lowest-id pending/expired jobs with `... ORDER BY id LIMIT n`, but with only a
plain `status` index Postgres walked the primary key from id=1, skipping the
entire prefix of already done/error rows before reaching pending ones. As `done`
grew (69k+), every lease became an O(done) scan — leasing crawled, the DB
saturated, and even /status (the queue GROUP BY count) stalled the agent.

- Migration 0070 adds two partial indexes over just the live slice: pending rows
  indexed by id (hot path), and leased rows by lease_expires_at (crash-recovery
  + orphan sweep). They stay tiny no matter how large the done/error history.
- lease() split into two phases so each uses a partial index: claim pending
  first (id-ordered, O(batch)); reclaim expired leases only when pending can't
  fill the batch. Same semantics (SKIP LOCKED, attempts++, expired reclaim).
- Model __table_args__ declares the indexes so ORM and schema agree.
- Test: a done-prefix at low ids must not stop the lease reaching pending.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

tests/test_gpu_jobs.py

@@ -141,6 +141,22 @@ async def test_lease_claims_then_skips_when_held(db):
     assert again == []
 
 
+@pytest.mark.asyncio
+async def test_lease_skips_done_prefix(db):
+    # A long prefix of already-done jobs at the LOW ids must not stop the lease
+    # from reaching the pending ones (the O(done) scan bug this indexing fixes).
+    svc = GpuJobService(db)
+    for i in range(5):
+        j = await svc.enqueue((await _img(db, f"d{i}" * 32)).id, "ccip")
+        j.status = "done"
+    target = await svc.enqueue((await _img(db, "dz" * 32)).id, "ccip")  # highest id
+    await db.commit()
+
+    leased = await svc.lease("agent-1", batch_size=8)
+    await db.commit()
+    assert [j.id for j in leased] == [target.id]
+
+
 @pytest.mark.asyncio
 async def test_expired_lease_is_reclaimed(db):
     img = await _img(db, "c" * 64)