refactor(subscriptions): remove the dead backfill dry-run preview (#1281)

PreviewDialog.vue was orphaned — nothing mounted it — so the dry-run preview
was unreachable. Rather than wire it up, remove the whole chain: its unique
value (a capped 3-page count of what a backfill would grab) is low, and its
adjacent needs are already covered — auth validation by verify_source_credential,
and actually fetching recent posts by "Check now". Operator decision 2026-07-06.

Removed:
- frontend: PreviewDialog.vue + sources store previewSource()
- backend: POST /api/sources/<id>/preview route, download_backends.preview_source,
  IngestCore.preview() + its now-unused NativeIngestError import
- tests: the 3 ingester preview tests

Nothing else referenced the chain (verified). Shared campaign-resolution and
ledger helpers stay — they're used by run()/verify_source_credential.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

tests/test_patreon_ingester.py

@@ -448,42 +448,6 @@ async def test_backfill_budget_cut_returns_partial_with_progress(
     assert result.cursor == "CUR2"
 
 
-@pytest.mark.asyncio
-async def test_preview_counts_new_media_without_downloading(
-    source_id, sync_engine, tmp_path,
-):
-    """plan #708 B4: preview walks + counts media not already seen/dead, downloads
-    nothing, and samples only posts that have new media."""
-    m1, m2, m3 = _media("p1", 1), _media("p2", 1), _media("p2", 2)
-    # Seed m1 as already-seen → only p2's two media are "new".
-    factory = sessionmaker(sync_engine, expire_on_commit=False)
-    with factory() as s:
-        s.add(PatreonSeenMedia(source_id=source_id, filehash=_ledger_key(m1), post_id="p1"))
-        s.commit()
-    client = _FakeClient([(None, [("p1", [m1]), ("p2", [m2, m3])])])
-    downloader = _FakeDownloader(tmp_path)
-    ing = _ingester(sync_engine, tmp_path, client, downloader)
-
-    result = ing.preview(source_id, "c1")
-    assert result["total_new"] == 2          # p2's m2 + m3 (m1 already seen)
-    assert result["posts_scanned"] == 2
-    assert result["has_more"] is False
-    assert downloader.download_calls == 0     # dry-run — nothing downloaded
-    # The sample lists only posts WITH new media (p2), not the all-seen p1.
-    assert [row["new"] for row in result["sample"]] == [2]
-
-
-@pytest.mark.asyncio
-async def test_preview_page_limit_caps_walk(source_id, sync_engine, tmp_path):
-    """plan #708 B4: preview stops after page_limit pages and flags has_more."""
-    pages = [(f"CUR{i}", [(f"p{i}", [_media(f"p{i}", 1)])]) for i in range(6)]
-    ing = _ingester(sync_engine, tmp_path, _FakeClient(pages), _FakeDownloader(tmp_path))
-    result = ing.preview(source_id, "c1", page_limit=2)
-    assert result["pages_scanned"] == 2
-    assert result["posts_scanned"] == 2   # one post per page, 2 pages
-    assert result["has_more"] is True
-
-
 @pytest.mark.asyncio
 async def test_write_live_progress_updates_running_event(
     source_id, sync_engine, tmp_path, db,
@@ -714,21 +678,6 @@ async def test_gated_post_skipped_entirely_no_media_no_record(
     assert _count_ledger(sync_engine, source_id) == 2
 
 
-@pytest.mark.asyncio
-async def test_preview_excludes_gated_posts(source_id, sync_engine, tmp_path):
-    """#874 preview/apply parity (rule 93): the dry-run must not count a gated
-    post's blurred preview media as new, or it overstates a gated source's
-    backlog."""
-    gm = _media("gated", 1)
-    am = _media("open", 1)
-    client = _FakeClient([(None, [("gated", [gm]), ("open", [am])])], gated={"gated"})
-    ing = _ingester(sync_engine, tmp_path, client, _FakeDownloader(tmp_path))
-
-    preview = ing.preview(source_id, "c1")
-    assert preview["total_new"] == 1                       # only the open post
-    assert [s for s in preview["sample"] if s["title"] == "gated"] == []
-
-
 @pytest.mark.asyncio
 async def test_recapture_does_not_refetch_seen_media_missing_from_disk(
     source_id, sync_engine, tmp_path,