feat: share post to other apps

internal/repository/post_repo.go

@@ -228,6 +228,62 @@ func (r *PostRepo) collectPosts(ctx context.Context, total int, query string, ar
 	return posts, total, nil
 }
 
+// AddReshare records that `viewerID` reshared `postID`. Idempotent —
+// repeated calls with the same pair are no-ops (the PRIMARY KEY makes
+// the second insert a conflict, which we explicitly ignore). Returns
+// nil on success regardless of whether a row was actually inserted.
+func (r *PostRepo) AddReshare(ctx context.Context, postID, viewerID string) error {
+	const q = `
+		INSERT INTO post_reshares (post_id, user_id)
+		VALUES ($1, $2)
+		ON CONFLICT (post_id, user_id) DO NOTHING`
+	if _, err := r.pg.Pool.Exec(ctx, q, postID, viewerID); err != nil {
+		return fmt.Errorf("add reshare: %w", err)
+	}
+	return nil
+}
+
+// RemoveReshare deletes the reshare relationship. Idempotent — deleting
+// a row that doesn't exist returns nil rather than ErrPostNotFound,
+// since "I'm not subscribed" is the desired end state either way.
+func (r *PostRepo) RemoveReshare(ctx context.Context, postID, viewerID string) error {
+	const q = `DELETE FROM post_reshares WHERE post_id = $1 AND user_id = $2`
+	if _, err := r.pg.Pool.Exec(ctx, q, postID, viewerID); err != nil {
+		return fmt.Errorf("remove reshare: %w", err)
+	}
+	return nil
+}
+
+// ReshareStatesFor returns a set of post IDs that `viewerID` has
+// reshared, filtered to the supplied `postIDs`. Same shape as
+// ImagesFor — one batched query so a feed render doesn't N+1 the
+// reshare table. Returns an empty map when `viewerID` is "" (anonymous
+// viewer can't have reshared anything).
+func (r *PostRepo) ReshareStatesFor(ctx context.Context, viewerID string, postIDs []string) (map[string]bool, error) {
+	out := make(map[string]bool, len(postIDs))
+	if viewerID == "" || len(postIDs) == 0 {
+		return out, nil
+	}
+	rows, err := r.pg.Pool.Query(ctx, `
+		SELECT post_id
+		FROM post_reshares
+		WHERE user_id = $1 AND post_id = ANY($2)`,
+		viewerID, postIDs,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("query reshare states: %w", err)
+	}
+	defer rows.Close()
+	for rows.Next() {
+		var id string
+		if err := rows.Scan(&id); err != nil {
+			return nil, fmt.Errorf("scan reshare state: %w", err)
+		}
+		out[id] = true
+	}
+	return out, rows.Err()
+}
+
 // ImagesFor returns a map post_id → [images...] for a batch of post IDs in
 // a single query so we don't N+1 the feed endpoint. Exported because the
 // CollectionService needs to hydrate images onto posts that come back