feat: add post like

migrations/0030_create_post_likes.down.sql

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+DROP TABLE IF EXISTS post_likes;
+ALTER TABLE posts DROP COLUMN IF EXISTS likes_count;

migrations/0030_create_post_likes.up.sql

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+-- Post likes.
+--
+-- `post_likes` is the membership table (one row per user-per-post like),
+-- written synchronously so a viewer's like state and the "liked by" list are
+-- immediately consistent. The UNIQUE(post_id, user_id) makes liking idempotent.
+--
+-- `posts.likes_count` is a denormalized counter that is NOT updated on every
+-- like (that hot-row UPDATE doesn't scale for popular posts). Instead each
+-- like/unlike nudges a per-post delta in Redis, and a background worker folds
+-- the deltas into this column every 15 minutes. Reads return
+-- likes_count + the still-pending Redis delta, so the displayed total is
+-- always current.
+ALTER TABLE posts ADD COLUMN IF NOT EXISTS likes_count INTEGER NOT NULL DEFAULT 0;
+
+CREATE TABLE IF NOT EXISTS post_likes (
+    id         UUID         PRIMARY KEY DEFAULT gen_random_uuid(),
+    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(),
+    UNIQUE (post_id, user_id)
+);
+
+-- Listing "who liked this post", newest first.
+CREATE INDEX IF NOT EXISTS post_likes_post_idx ON post_likes (post_id, created_at DESC);
+-- "Have I liked these posts?" — viewer-state hydration across a feed.
+CREATE INDEX IF NOT EXISTS post_likes_user_idx ON post_likes (user_id);

migrations/0031_notifications_like_aggregate.down.sql

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+DROP INDEX IF EXISTS notifications_post_like_uq;
+ALTER TABLE notifications DROP COLUMN IF EXISTS actor_count;
+ALTER TABLE notifications DROP COLUMN IF EXISTS post_id;

migrations/0031_notifications_like_aggregate.up.sql

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+-- 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';