feat: add post like

migrations/0030_create_post_likes.up.sql

@@ -0,0 +1,26 @@
+-- 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);