feat: post reshare enhancements

migrations/0014_create_post_reshare_images.up.sql

@@ -0,0 +1,35 @@
+-- Per-reshare background image overrides.
+--
+-- Previously every viewer of a reshare saw the canonical post_images row —
+-- so when either the original author or any resharer "Updated background"
+-- the change leaked across all timelines that referenced the post. This
+-- table separates "the post's background" from "this user's reshare of
+-- the post's background": authoring stays in post_images, resharing
+-- stores its own snapshot here.
+--
+-- At reshare time we copy the post's current post_images into this table
+-- so the resharer's feed renders identically to the author's. After that
+-- moment the two are independent — the resharer's background lives only
+-- here, the original author's lives only in post_images, and changes to
+-- one never touch the other.
+--
+-- Cascade: the FK targets the (post_id, user_id) primary key on
+-- post_reshares. Every path that removes a reshare relationship (the
+-- author deletes the post, the resharer un-reshares, either user is
+-- deleted) drops the post_reshares row, which cascades the overrides
+-- away. The user's stated rule — "should not be in sync except deletion"
+-- — is preserved end-to-end without any extra application code.
+
+CREATE TABLE IF NOT EXISTS post_reshare_images (
+    id          UUID         PRIMARY KEY DEFAULT gen_random_uuid(),
+    post_id     UUID         NOT NULL,
+    user_id     UUID         NOT NULL,
+    url         TEXT         NOT NULL,
+    position    INT          NOT NULL DEFAULT 0,
+    created_at  TIMESTAMPTZ  NOT NULL DEFAULT now(),
+    FOREIGN KEY (post_id, user_id)
+        REFERENCES post_reshares (post_id, user_id) ON DELETE CASCADE
+);
+
+CREATE INDEX IF NOT EXISTS idx_post_reshare_images_lookup
+    ON post_reshare_images (post_id, user_id, position);