fix: phase 3 stops importing at a deadline and leaves the rest for the next chunk

A Discord recapture chunk on #banana-land was killed at the 1350s soft limit
twice (events 92354 on 2026-09-28 and 92623 on 2026-09-29). The walk's
budget (CHUNK_TOTAL_SECONDS, #4407) assumed 2.5s per import; measured on
those chunks, a real import of the source's doodles took about 5s, so ~290
imports the walk had planned to fit took about twice the time reserved.

- PHASE3_DEADLINE_SECONDS (1200s from task start): on native platforms
  phase 3 starts no import past it. The files left stay on disk with no
  record and are not marked seen; the next chunk resumes on the page that
  holds the first of them and imports them through the existing "on disk
  but never imported" path. A walk that reached its end becomes PARTIAL so
  the backfill is not marked complete while files wait.
- DownloadResult.written_cursors records each written file's page cursor;
  mark_seen_after_import takes skip_paths.
- PHASE3_IMPORT_SECONDS 2.5 -> 5.0, the slow end of what was measured, so
  the walk rarely leans on the deadline.

Task #4493.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_patreon_ingester.py

@@ -984,6 +984,31 @@ async def test_recapture_imports_a_seen_file_nothing_imported(
     assert downloader.download_calls == 0
 
 
+@pytest.mark.asyncio
+async def test_files_phase_3_leaves_unimported_stay_unmarked(
+    source_id, sync_engine, tmp_path,
+):
+    """Phase 3 can stop importing at its deadline. Each written file names the
+    page it came from, so the next chunk can resume there; the files left keep
+    no ledger mark, so that walk meets them on disk and imports them."""
+    m1, m2 = _media("p1", 1), _media("p2", 1)
+    pages = [("CUR1", [("p1", [m1])]), ("CUR2", [("p2", [m2])])]
+    ing = _ingester(sync_engine, tmp_path, _FakeClient(pages), _FakeDownloader(tmp_path))
+    result = ing.run(source_id=source_id, campaign_id="c1", artist_slug="ingest",
+                     url="https://patreon.com/ingest", mode="backfill")
+    assert result.written_cursors == ["CUR1", "CUR2"]
+
+    result.mark_seen_after_import(skip_paths={str(tmp_path / "p2_1.jpg")})
+    assert _count_ledger(sync_engine, source_id) == 3   # m1 and both post records
+
+    ing2 = _ingester(sync_engine, tmp_path, _FakeClient(pages),
+                     _FakeDownloader(tmp_path, on_disk={_ledger_key(m1), _ledger_key(m2)}))
+    again = ing2.run(source_id=source_id, campaign_id="c1", artist_slug="ingest",
+                     url="https://patreon.com/ingest", mode="backfill")
+    assert again.written_paths == [str(tmp_path / "p2_1.jpg")]
+    assert "on disk but never imported: p2_1.jpg" in again.stdout
+
+
 # --- dead-letter ledger (plan #705 #7) ------------------------------------