feat(download): smarter backfill — time-boxed chunks, run-until-done (backend)

Plan #693. Large-catalog backfill (Anduo) no longer sprints to the timeout
wall and dies as an error each run. Builds on the cursor checkpoint (#689).

- Time-boxed chunks: BACKFILL_TIMEOUT_SECONDS(1170)→BACKFILL_CHUNK_SECONDS(600),
  far under the 1350 soft limit. Hitting it = normal chunk boundary (the
  TimeoutExpired path already captures partial output + the cursor), not a
  near-wall death.
- Run-until-done state machine driven by config_overrides[_backfill_state]
  (running/complete/stalled). A running backfill auto-continues in chunks
  across ticks until gallery-dl exits cleanly (rc=0 = reached the bottom →
  'complete'); a safety-cap (BACKFILL_MAX_CHUNKS=200) + the #689 stall-guard
  pause a pathological walk as 'stalled'. Replaces the N-runs counter
  (backfill_runs_remaining repurposed as the cap countdown).
- Progress, not error: a chunk that timed out but advanced (cursor moved
  and/or files written) is reclassified TIMEOUT→PARTIAL (status 'ok').
- Retry storm tamed: gallery-dl retries 3→2, downloader timeout 120→60s, so
  one stuck CDN file fails in ~1-2 min not ~10 (Anduo #40838).
- API: POST /sources/{id}/backfill now takes {action: start|stop}; service
  start_backfill/stop_backfill; new enabled sources auto-arm run-until-done;
  source dict exposes backfill_state + backfill_chunks.

Frontend (Start/Stop control + state badge) lands in the next push.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_api_sources.py

@@ -199,7 +199,7 @@ async def test_list_derives_next_check_at_when_last_checked_set(
 
 
 @pytest.mark.asyncio
-async def test_backfill_endpoint_arms_source(client, artist, db):
+async def test_backfill_endpoint_start_and_stop(client, artist, db):
     src = Source(
         artist_id=artist.id, platform="patreon",
         url="https://patreon.com/alice-backfill", enabled=True,
@@ -208,18 +208,21 @@ async def test_backfill_endpoint_arms_source(client, artist, db):
     await db.commit()
     sid = src.id
 
-    resp = await client.post(f"/api/sources/{sid}/backfill", json={"runs": 5})
+    resp = await client.post(f"/api/sources/{sid}/backfill", json={"action": "start"})
     assert resp.status_code == 200
     body = await resp.get_json()
-    assert body["backfill_runs_remaining"] == 5
+    assert body["backfill_state"] == "running"
 
-    # GET reflects the new state.
+    # GET reflects the running state.
     one = await client.get(f"/api/sources/{sid}")
-    assert (await one.get_json())["backfill_runs_remaining"] == 5
+    assert (await one.get_json())["backfill_state"] == "running"
+
+    stopped = await client.post(f"/api/sources/{sid}/backfill", json={"action": "stop"})
+    assert (await stopped.get_json())["backfill_state"] is None
 
 
 @pytest.mark.asyncio
-async def test_backfill_endpoint_defaults_to_three(client, artist, db):
+async def test_backfill_endpoint_defaults_to_start(client, artist, db):
     src = Source(
         artist_id=artist.id, platform="patreon",
         url="https://patreon.com/alice-backfill-default", enabled=True,
@@ -228,26 +231,22 @@ async def test_backfill_endpoint_defaults_to_three(client, artist, db):
     await db.commit()
     resp = await client.post(f"/api/sources/{src.id}/backfill", json={})
     body = await resp.get_json()
-    assert body["backfill_runs_remaining"] == 3
+    assert body["backfill_state"] == "running"
 
 
 @pytest.mark.asyncio
-async def test_backfill_endpoint_rejects_out_of_range(client, artist, db):
+async def test_backfill_endpoint_rejects_bad_action(client, artist, db):
     src = Source(
         artist_id=artist.id, platform="patreon",
         url="https://patreon.com/alice-backfill-bad", enabled=True,
     )
     db.add(src)
     await db.commit()
-    bad = await client.post(f"/api/sources/{src.id}/backfill", json={"runs": 0})
+    bad = await client.post(f"/api/sources/{src.id}/backfill", json={"action": "nope"})
     assert bad.status_code == 400
-    too_big = await client.post(
-        f"/api/sources/{src.id}/backfill", json={"runs": 99}
-    )
-    assert too_big.status_code == 400
 
 
 @pytest.mark.asyncio
 async def test_backfill_endpoint_404_when_source_missing(client):
-    resp = await client.post("/api/sources/999999/backfill", json={"runs": 3})
+    resp = await client.post("/api/sources/999999/backfill", json={"action": "start"})
     assert resp.status_code == 404