feat: add post mention

internal/repository/post_repo.go

@@ -171,6 +171,32 @@ func (r *PostRepo) ListByHandle(ctx context.Context, handle string, limit, offse
 	return r.ListByUser(ctx, userID, limit, offset)
 }
 
+// ListMentioningByHandle returns posts that tag the given user (handle),
+// newest first — the "Mentions" profile tab. Same shape as the profile feed.
+func (r *PostRepo) ListMentioningByHandle(ctx context.Context, handle string, limit, offset int) ([]model.Post, int, error) {
+	var userID string
+	if err := r.pg.Pool.QueryRow(ctx, `SELECT id FROM users WHERE handle = $1`, handle).Scan(&userID); err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return []model.Post{}, 0, nil
+		}
+		return nil, 0, fmt.Errorf("lookup user by handle: %w", err)
+	}
+
+	var total int
+	if err := r.pg.Pool.QueryRow(ctx,
+		`SELECT count(*) FROM posts p JOIN post_mentions pm ON pm.post_id = p.id WHERE pm.user_id = $1`,
+		userID,
+	).Scan(&total); err != nil {
+		return nil, 0, fmt.Errorf("count mentioning posts: %w", err)
+	}
+	q := `SELECT ` + postSelectColumns + postSelectFrom + `
+		JOIN post_mentions pm ON pm.post_id = p.id
+		WHERE pm.user_id = $1
+		ORDER BY p.created_at DESC, p.id
+		LIMIT $2 OFFSET $3`
+	return r.collectPosts(ctx, total, q, userID, limit, offset)
+}
+
 // ListByUser returns the user's profile feed — every post they own,
 // authored or reshared. Reshares are interleaved naturally because
 // they're stored as real rows; sort by created_at gets the right order