feat: deduplicate feed items by guid

Use ROW_NUMBER() window function to filter out duplicate articles
with the same guid across different feeds. This ensures each guid
appears only once in unread/read article lists.

- Add idx_articles_guid index for PARTITION BY performance
- Modify GetUnreadArticles and GetReadArticles queries with CTE

1 files changed, 2 insertions, 0 deletions

diff --git a/backend/db/migrations/005_add_guid_index.sql b/backend/db/migrations/005_add_guid_index.sql
new file mode 100644
index 0000000..a653d79
--- /dev/null
+++ b/backend/db/migrations/005_add_guid_index.sql
@@ -0,0 +1,2 @@
+-- Add index on guid for deduplication queries
+CREATE INDEX IF NOT EXISTS idx_articles_guid ON articles(guid);