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_download_source_task.py

@@ -55,6 +55,15 @@ def test_the_native_chunk_leaves_room_to_tear_down_before_the_soft_limit():
     assert CHUNK_TOTAL_SECONDS <= DOWNLOAD_SOFT_TIME_LIMIT - 120
 
 
+def test_phase_3_stops_importing_with_room_to_finalize():
+    """Past its deadline phase 3 starts no import; what remains after it (the
+    import in hand, post records, the event) has to fit before the soft limit."""
+    from backend.app.services.ingest_core import PHASE3_DEADLINE_SECONDS
+    from backend.app.tasks.download import DOWNLOAD_SOFT_TIME_LIMIT
+
+    assert PHASE3_DEADLINE_SECONDS <= DOWNLOAD_SOFT_TIME_LIMIT - 120
+
+
 def test_decorated_limits_match_module_constants():
     """The @celery.task decorator must use the audited constants, not
     drifted literals."""