feat: add post mention

migrations/0033_create_post_mentions.down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS post_mentions;

migrations/0033_create_post_mentions.up.sql

@@ -0,0 +1,14 @@
+-- Post mentions — users tagged on a post (separate from the locked caption).
+-- Membership table; the count is small and edited rarely, so it's derived at
+-- read time (no denormalized counter / worker needed).
+CREATE TABLE IF NOT EXISTS post_mentions (
+    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)
+);
+
+-- List a post's mentions (insertion order) + hydrate mention counts across a feed.
+CREATE INDEX IF NOT EXISTS post_mentions_post_idx ON post_mentions (post_id, created_at);
+CREATE INDEX IF NOT EXISTS post_mentions_user_idx ON post_mentions (user_id);