feat: post reshare enhancements

migrations/0015_reshares_are_real_posts.down.sql

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+-- Reverting this requires materialising the new reshared posts back into
+-- the post_reshares + post_reshare_images join model and then dropping
+-- the column. Documented but not automated — run manually if a rollback
+-- is ever actually needed.
+--
+-- Step 1: recreate the old tables.
+-- Step 2: insert one row per reshare back into post_reshares.
+-- Step 3: copy each reshare's post_images back into post_reshare_images.
+-- Step 4: delete the materialised reshare rows from `posts`.
+-- Step 5: drop the column.
+
+ALTER TABLE posts DROP COLUMN IF EXISTS reshared_from_post_id;