added ratings and updated ebook struct

backend/migrations/002_add_ratings.up.sql

@@ -0,0 +1,14 @@
+-- Add ebook ratings functionality
+CREATE TABLE ebook_ratings (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    ebook_id UUID NOT NULL REFERENCES ebooks(id) ON DELETE CASCADE,
+    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    rating INTEGER NOT NULL CHECK (rating >= 1 AND rating <= 5),
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    updated_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    UNIQUE(ebook_id, user_id)
+);
+
+-- Create indexes for better query performance
+CREATE INDEX idx_ebook_ratings_ebook_id ON ebook_ratings(ebook_id);
+CREATE INDEX idx_ebook_ratings_user_id ON ebook_ratings(user_id);