feat: add feed sidebar and cursor-based pagination

Add a feed sidebar to /unread and /read pages for filtering articles by
feed, and replace the fixed 100-article limit with cursor-based
pagination using a "Load more" button.

Backend:
- Add PageInfo, ArticleConnection types and pagination args to GraphQL
- Replace GetUnreadArticles/GetReadArticles with parameterized queries
- Add GetFeedUnreadCounts query and composite index
- Add shared pagination helper in resolver

Frontend:
- Add FeedSidebar component with unread count badges
- Add usePaginatedArticles hook for cursor-based fetching
- Update ArticleList with Load more button and single-feed mode
- Use ?feed=<id> query parameter for feed filtering

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

1 files changed, 2 insertions, 0 deletions

diff --git a/backend/db/schema.sql b/backend/db/schema.sql
index 07ac72d..2596c84 100644
--- a/backend/db/schema.sql
+++ b/backend/db/schema.sql
@@ -37,3 +37,5 @@ CREATE INDEX IF NOT EXISTS idx_articles_is_read ON articles(is_read);
 CREATE INDEX IF NOT EXISTS idx_articles_guid ON articles(guid);
 
 CREATE INDEX IF NOT EXISTS idx_feeds_user_id ON feeds(user_id);
+
+CREATE INDEX IF NOT EXISTS idx_articles_feed_read_id ON articles(feed_id, is_read, id DESC);