feat: add post like

migrations/0031_notifications_like_aggregate.up.sql

@@ -0,0 +1,14 @@
+-- Aggregated like notifications.
+--
+-- Likes are high-volume, so instead of one notification row per like we keep a
+-- single row per (recipient, post) and aggregate: actor_id holds the most
+-- recent liker, actor_count how many distinct likers have aggregated into it.
+-- The partial unique index lets the like path UPSERT that one row (bumping the
+-- count, refreshing the actor, re-marking unread) — so the activity list shows
+-- "X and N others liked your post", not a flood.
+ALTER TABLE notifications ADD COLUMN IF NOT EXISTS post_id UUID REFERENCES posts(id) ON DELETE CASCADE;
+ALTER TABLE notifications ADD COLUMN IF NOT EXISTS actor_count INTEGER NOT NULL DEFAULT 1;
+
+CREATE UNIQUE INDEX IF NOT EXISTS notifications_post_like_uq
+    ON notifications (user_id, post_id)
+    WHERE kind = 'post_like';