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/models/import_settings.py

@@ -158,6 +158,31 @@ class ImportSettings(Base):
         server_default="true",
     )
 
+    # The unified card (#4402). A linked Discord drop is NOT absorbed into its
+    # teaser — it keeps its own post, date and provenance, and the teaser's card
+    # shows it by REFERENCE. Operator, 2026-09-24: *"discord 'posts' land as
+    # normal and only hidden from the post view they're posted the same day."*
+    #
+    # So the drop's own card leaves the feed only when it sits within this many
+    # hours of the teaser that references it — the adjacency that reads as the
+    # same thing twice. Hours rather than a calendar day: a teaser at 23:00 and
+    # its drop at 01:00 are one release, and "the same day" has no timezone
+    # the server can know.
+    discord_link_fold_hours: Mapped[float] = mapped_column(
+        Float, nullable=False, default=24.0,
+        server_default="24",
+    )
+    # How far from the teaser the card reaches for the rest of a piece's
+    # variants — the wips, alts and censor passes a creator trickles out under
+    # one working name (#4401). Measured on artist 8: named families spread a
+    # median 5 days and up to 44, while every name collision found spreads
+    # over 500. A reference, not a regrouping, so a generous value costs one
+    # extra thumbnail at worst — never a post moved or hidden.
+    discord_family_window_days: Mapped[float] = mapped_column(
+        Float, nullable=False, default=60.0,
+        server_default="60",
+    )
+
     # #830 off-platform file-host downloads — per-host enable lever (default on,
     # rule #26). Column names are extdl_<host>_enabled so the worker reads them
     # via getattr(settings, f"extdl_{host}_enabled", True).