feat: post reshare enhancements

migrations/0015_reshares_are_real_posts.up.sql

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+-- Reshares become real `posts` rows.
+--
+-- Previously a reshare was a join-table relationship between (post, user)
+-- that pointed at the original author's canonical row. Likes, comments,
+-- and (most recently) background overrides all had to be plumbed through
+-- a parallel "per-resharer" path to avoid leaking across timelines. The
+-- next set of social features (likes, comments) would multiply that
+-- complexity again.
+--
+-- This migration collapses that model: every reshare is its own row in
+-- `posts`, owned by the resharer, with a `reshared_from_post_id` pointer
+-- back to the canonical original. Likes and comments will naturally hang
+-- off post.id and diverge for free; backgrounds are already per-row in
+-- post_images so the per-reshare override table goes away.
+--
+-- Deletion still cascades: the FK uses ON DELETE CASCADE on
+-- reshared_from_post_id, so an author deleting their post nukes every
+-- reshare of it (including their post_images via the existing FK).
+
+-- 1. New self-referential column. Nullable: NULL on canonical posts,
+--    set on reshares.
+ALTER TABLE posts
+    ADD COLUMN IF NOT EXISTS reshared_from_post_id UUID
+        REFERENCES posts(id) ON DELETE CASCADE;
+
+CREATE INDEX IF NOT EXISTS idx_posts_reshared_from
+    ON posts (reshared_from_post_id)
+    WHERE reshared_from_post_id IS NOT NULL;
+
+-- Composite index used by ReshareStatesFor — given a viewer + a batch of
+-- canonical post ids, "which of these has the viewer reshared?".
+CREATE INDEX IF NOT EXISTS idx_posts_user_reshared_from
+    ON posts (user_id, reshared_from_post_id)
+    WHERE reshared_from_post_id IS NOT NULL;
+
+-- 2. Backfill: each row in post_reshares becomes a real `posts` row,
+--    copying the canonical post's textual fields and the timestamp the
+--    reshare originally happened. The new row is owned by the resharer.
+--
+--    Skipped if post_reshares doesn't exist (clean install) — the
+--    DO block uses to_regclass so a fresh database doesn't error.
+DO $$
+BEGIN
+    IF to_regclass('public.post_reshares') IS NOT NULL THEN
+        INSERT INTO posts (
+            id, user_id, caption, source_url, source_title,
+            created_at, updated_at, reshared_from_post_id
+        )
+        SELECT
+            gen_random_uuid(),
+            r.user_id,
+            p.caption,
+            p.source_url,
+            p.source_title,
+            r.created_at,
+            r.created_at,
+            p.id
+        FROM post_reshares r
+        JOIN posts p ON p.id = r.post_id;
+    END IF;
+END$$;
+
+-- 3. Carry the per-resharer image overrides over. Each override row maps
+--    to the newly-inserted post via (reshared_from_post_id, user_id).
+DO $$
+BEGIN
+    IF to_regclass('public.post_reshare_images') IS NOT NULL THEN
+        INSERT INTO post_images (post_id, url, position)
+        SELECT np.id, ri.url, ri.position
+        FROM post_reshare_images ri
+        JOIN posts np
+            ON np.reshared_from_post_id = ri.post_id
+           AND np.user_id = ri.user_id;
+    END IF;
+END$$;
+
+-- 4. Old tables are no longer referenced anywhere — drop them so the
+--    schema mirrors the new model.
+DROP TABLE IF EXISTS post_reshare_images;
+DROP TABLE IF EXISTS post_reshares;