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

alembic/versions/0110_unified_post_card.py

@@ -0,0 +1,59 @@
+"""The unified post card — fold window, family window, and who linked a pair.
+
+Milestone 388, #4402 and #4401. A Patreon teaser's card shows the Discord drop
+it announced, and the rest of that piece's variants, by REFERENCE: nothing is
+absorbed, nothing changes owner, and every Discord post keeps its own place.
+
+Three columns:
+
+* `import_settings.discord_link_fold_hours` — a linked drop leaves the feed
+  only when it is this close to its teaser (the same release, shown twice).
+* `import_settings.discord_family_window_days` — how far from the teaser the
+  card reaches for variants. 60 is measured: named families spread up to 44
+  days on artist 8, every collision found over 500.
+* `post_association.linked_by` — "fc" or "operator", so a link FC made by
+  itself can say so on the card and offer the undo the operator asked for.
+
+Revision ID: 0110
+Revises: 0109
+Create Date: 2026-09-24
+
+"""
+import sqlalchemy as sa
+from alembic import op
+
+revision = "0110"
+down_revision = "0109"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.add_column(
+        "import_settings",
+        sa.Column(
+            "discord_link_fold_hours",
+            sa.Float(),
+            nullable=False,
+            server_default=sa.text("24"),
+        ),
+    )
+    op.add_column(
+        "import_settings",
+        sa.Column(
+            "discord_family_window_days",
+            sa.Float(),
+            nullable=False,
+            server_default=sa.text("60"),
+        ),
+    )
+    op.add_column(
+        "post_association",
+        sa.Column("linked_by", sa.String(length=16), nullable=True),
+    )
+
+
+def downgrade():
+    op.drop_column("post_association", "linked_by")
+    op.drop_column("import_settings", "discord_family_window_days")
+    op.drop_column("import_settings", "discord_link_fold_hours")