fix: a non-image file lands on the source it was downloaded for, and the undated shells it left are folded into their real posts (#4435)

The attachment path looked the post's source up by (artist, platform),
taking the artist's lowest-id source. A Discord artist has one source per
channel, so every archive or pdf from a later channel became an undated
second post on the first channel's source. _post_for_sidecar now takes
the downloading source, as upsert_post_record and _apply_sidecar already
did. Migration 0114 re-points each shell's attachments to its dated twin
and deletes the shell.

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

alembic/versions/0114_fold_misfiled_attachment_posts.py

@@ -0,0 +1,79 @@
+"""Fold the undated shell posts a misfiled attachment created into the real post.
+
+#4435. A non-image file (an archive, a pdf) downloaded for one of an artist's
+Discord channels was filed under the artist's FIRST Discord source: the
+attachment path looked the source up by (artist, platform), which takes the
+lowest id. That created an undated, url-less post there holding only the
+attachment, and the message's real post record then created the dated post
+under the right source. The importer now uses the source it was downloading
+for; this repairs the pairs it left.
+
+A shell is folded only when all of this holds:
+
+* it has no date and no url, and nothing synthesized it;
+* another post of the same artist, platform and external id HAS a date;
+* no image is linked to the shell (it held an attachment and nothing else).
+
+Its attachments move to the dated post, dropping any the dated post already
+has (same sha256, which the per-post unique forbids twice), and the shell is
+deleted. A shell with no dated twin is left alone: which channel it belongs to
+is not recorded anywhere but the file name. The downgrade does nothing.
+
+Revision ID: 0114
+Revises: 0113
+Create Date: 2026-09-25
+
+"""
+import sqlalchemy as sa
+from alembic import op
+
+revision = "0114"
+down_revision = "0113"
+branch_labels = None
+depends_on = None
+
+_PAIRS = """
+    SELECT DISTINCT ON (shell.id) shell.id AS shell_id, real.id AS real_id
+    FROM post shell
+    JOIN source ss ON ss.id = shell.source_id
+    JOIN post real
+      ON real.artist_id = shell.artist_id
+     AND real.external_post_id = shell.external_post_id
+     AND real.id <> shell.id
+     AND real.post_date IS NOT NULL
+    JOIN source rs ON rs.id = real.source_id AND rs.platform = ss.platform
+    WHERE shell.post_date IS NULL
+      AND shell.post_url IS NULL
+      AND shell.synthesized_by IS NULL
+      AND NOT EXISTS (SELECT 1 FROM image_provenance ip WHERE ip.post_id = shell.id)
+    ORDER BY shell.id, real.id
+"""
+
+
+def fold_misfiled_attachment_posts(conn) -> int:
+    """The data step, on a plain connection, so a test can run it directly.
+    Returns how many shells were folded."""
+    pairs = conn.execute(sa.text(_PAIRS)).all()
+    for shell_id, real_id in pairs:
+        conn.execute(sa.text("""
+            DELETE FROM post_attachment pa
+            WHERE pa.post_id = :shell
+              AND EXISTS (
+                SELECT 1 FROM post_attachment keep
+                WHERE keep.post_id = :real AND keep.sha256 = pa.sha256
+              )
+        """), {"shell": shell_id, "real": real_id})
+        conn.execute(
+            sa.text("UPDATE post_attachment SET post_id = :real WHERE post_id = :shell"),
+            {"shell": shell_id, "real": real_id},
+        )
+        conn.execute(sa.text("DELETE FROM post WHERE id = :shell"), {"shell": shell_id})
+    return len(pairs)
+
+
+def upgrade() -> None:
+    fold_misfiled_attachment_posts(op.get_bind())
+
+
+def downgrade() -> None:
+    pass