feat: a teaser's card references the drop it announced and the piece's variants (4402, 4401)

The operator's problem: a Patreon teaser is a pointer, and its card showed the
censored crop plus a text link while the content it pointed at sat on another
card. The fix is a REFERENCE, not an absorption: "the nested items on the
unified post are a duplicate or reference of existing content". Nothing is
written. Discord posts keep their own rows, dates and places in the feed.

- post_unification: for each teaser with a linked association, the drop's
  images and text, plus its variant family: Discord images sharing the
  seed's gated LEADING working name, or a phash near-duplicate, within a
  window of the teaser. One hop only, oldest first.
- Measured on artist 8 before writing it: of 121 message pairs 2-60 days
  apart that share a gated token, 106 share the leading name and all read
  as real families. Of the 15 sharing only a trailing word, 14 are sibling
  pieces and one is a plain collision (`bottom`, 56 days). The family cap
  is 8, not the pairing cap of 6, because `tentacooler` and `0-k1` (6 posts
  each) are real families.
- The feed drops a linked drop's own card only within discord_link_fold_hours
  of its teaser (default 24): "only hidden from the post view they're posted
  the same day". Older referenced posts stay where they landed.
- post_association.linked_by records whether FC or a person made the link,
  so the card can say so. Undo is the existing dismiss.
- `image0` (gallery-dl's fallback name) becomes a stopword.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

tests/test_api_settings_downloader.py

@@ -135,3 +135,34 @@ async def test_auto_linking_refuses_a_non_boolean(client):
         "/api/settings/import", json={"discord_link_auto": "yes"}
     )
     assert resp.status_code == 400
+
+
+# --- #4402: the unified card's two windows ---------------------------------
+
+
+@pytest.mark.asyncio
+async def test_the_unified_card_windows_ship_with_their_measured_defaults(client):
+    """24h is "the same day" without a timezone the server cannot know; 60 days
+    is the measured family boundary (named families up to 44d on artist 8,
+    every collision over 500)."""
+    resp = await client.get("/api/settings/import")
+    body = await resp.get_json()
+    assert body["discord_link_fold_hours"] == 24
+    assert body["discord_family_window_days"] == 60
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("key", ["discord_link_fold_hours", "discord_family_window_days"])
+async def test_zero_turns_a_unified_card_window_off(client, key):
+    """Zero is a real answer for both: fold nothing, reference no variants."""
+    resp = await client.patch("/api/settings/import", json={key: 0})
+    assert resp.status_code == 200
+    assert (await resp.get_json())[key] == 0
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("key", ["discord_link_fold_hours", "discord_family_window_days"])
+@pytest.mark.parametrize("bad", [-1, "24", True])
+async def test_a_unified_card_window_refuses_nonsense(client, key, bad):
+    resp = await client.patch("/api/settings/import", json={key: bad})
+    assert resp.status_code == 400