feat(reader): chapter comments + readers-this-week count

- CommentsSection now accepts a chapter prop and scopes comments to that chapter
- Chapter reader page mounts CommentsSection with current chapter number
- Book detail page shows rolling 7-day unique reader count badge
- API GET/POST pass chapter param; pocketbase listComments filters by chapter

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/routes/api/comments/[slug]/+server.ts

@@ -21,9 +21,10 @@ export const GET: RequestHandler = async ({ params, url, locals }) => {
 	const { slug } = params;
 	const sortParam = url.searchParams.get('sort') ?? 'new';
 	const sort: CommentSort = sortParam === 'top' ? 'top' : 'new';
+	const chapter = parseInt(url.searchParams.get('chapter') ?? '0', 10) || 0;
 
 	try {
-		const topLevel = await listComments(slug, sort);
+		const topLevel = await listComments(slug, sort, chapter);
 
 		// Fetch replies for all top-level comments in parallel
 		const repliesPerComment = await Promise.all(topLevel.map((c) => listReplies(c.id)));
@@ -75,7 +76,7 @@ export const POST: RequestHandler = async ({ params, request, locals }) => {
 	if (!locals.user) error(401, 'Login required to comment');
 
 	const { slug } = params;
-	let body: { body?: string; parent_id?: string };
+	let body: { body?: string; parent_id?: string; chapter?: number };
 	try {
 		body = await request.json();
 	} catch {
@@ -86,8 +87,8 @@ export const POST: RequestHandler = async ({ params, request, locals }) => {
 	if (!text) error(400, 'Comment body is required');
 	if (text.length > 2000) error(400, 'Comment is too long (max 2000 characters)');
 
-	// Enforce 1-level depth: parent_id must be a top-level comment
 	const parentId = body.parent_id?.trim() || undefined;
+	const chapter = typeof body.chapter === 'number' ? body.chapter : 0;
 
 	try {
 		const comment = await createComment(
@@ -95,7 +96,8 @@ export const POST: RequestHandler = async ({ params, request, locals }) => {
 			text,
 			locals.user.id,
 			locals.user.username,
-			parentId
+			parentId,
+			chapter
 		);
 		return json(comment, { status: 201 });
 	} catch (e) {