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

backend/app/api/settings.py

@@ -45,6 +45,8 @@ _EDITABLE_FIELDS = (
     "discord_link_threshold",
     "discord_link_window_hours",
     "discord_link_auto",
+    "discord_link_fold_hours",
+    "discord_family_window_days",
     "extdl_mega_enabled",
     "extdl_gdrive_enabled",
     "extdl_mediafire_enabled",
@@ -182,6 +184,12 @@ async def update_import_settings():
             return jsonify(
                 {"error": "discord_link_window_hours must be a positive number"}
             ), 400
+    # Zero is meaningful for both: fold nothing, or reference no variants.
+    for key in ("discord_link_fold_hours", "discord_family_window_days"):
+        if key in body:
+            v = body[key]
+            if not isinstance(v, (int, float)) or isinstance(v, bool) or v < 0:
+                return jsonify({"error": f"{key} must be a number >= 0"}), 400
     if "wip_title_tagging_enabled" in body and not isinstance(
         body["wip_title_tagging_enabled"], bool
     ):