feat: share post to other apps

migrations/0010_create_post_reshares.down.sql

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+DROP INDEX IF EXISTS idx_post_reshares_user_created;
+DROP TABLE IF EXISTS post_reshares;

migrations/0010_create_post_reshares.up.sql

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+-- Tracks which posts a user has reshared. Pure join table — no
+-- denormalised counters, no duplicate post rows. The act of resharing
+-- is a relationship between (post, user); the post itself stays in
+-- one canonical row owned by its original author.
+--
+-- This powers the post-card swap: posts a user has authored OR
+-- reshared show the "Send" (external share) button instead of
+-- "Reshare", because there's nothing left to bring onto the account.
+--
+-- Composite PK on (post_id, user_id) makes the relationship unique
+-- and gives us a covered lookup for the "did this viewer reshare this
+-- post?" check used in batched feed hydration.
+--
+-- Index on (user_id, created_at DESC) keeps "what has this user
+-- reshared, newest first?" cheap for the day we surface that on the
+-- profile feed.
+CREATE TABLE IF NOT EXISTS post_reshares (
+    post_id    UUID NOT NULL REFERENCES posts(id) ON DELETE CASCADE,
+    user_id    UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    PRIMARY KEY (post_id, user_id)
+);
+
+CREATE INDEX IF NOT EXISTS idx_post_reshares_user_created
+    ON post_reshares (user_id, created_at DESC);