feat: post reshare enhancements

migrations/0016_backfill_reshare_inherited_images.down.sql

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+-- This is a data-only backfill — there's no clean rollback because we
+-- can't distinguish post_images rows added by this migration from rows
+-- the resharer added afterwards. Intentional no-op; documented for the
+-- runbook only.
+SELECT 1;

migrations/0016_backfill_reshare_inherited_images.up.sql

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+-- Backfill the "reshares inherit the canonical's images" rule for any
+-- reshare row that landed in `posts` without an image set.
+--
+-- Two paths created such rows:
+--
+--   1. Reshares made BEFORE migration 0014 had no per-resharer image
+--      table to snapshot into, so when 0015 collapsed the old
+--      post_reshares model into real posts rows there was nothing for
+--      it to copy — those reshares now show with a blank background
+--      where the original author has a cover image.
+--
+--   2. Any future code path that creates a reshare row but forgets to
+--      copy the images. Defensive — current `Reshare` in
+--      internal/repository/post_repo.go does copy, but this migration
+--      makes the database self-healing if we ever regress.
+--
+-- The copy is scoped to reshares with ZERO images of their own so we
+-- never overwrite a resharer's explicit customization (including an
+-- explicit "no background" — though that path didn't exist at the time
+-- the affected rows were created, the guard keeps us safe for any
+-- future re-runs of equivalent backfills).
+INSERT INTO post_images (post_id, url, position)
+SELECT r.id, i.url, i.position
+FROM posts r
+JOIN post_images i ON i.post_id = r.reshared_from_post_id
+WHERE r.reshared_from_post_id IS NOT NULL
+  AND NOT EXISTS (
+      SELECT 1 FROM post_images WHERE post_id = r.id
+  );