feat: add database support for multiple user ebook folders

- Add ebook_folder_path column to users table (migration 004)
- Create user_ebook_folders table for multiple folders per user (migration 005)
- Remove old ebook_folder_path column from users table (migration 006)
- Add unique constraint to prevent duplicate folder paths per user

backend/migrations/004_add_ebook_folder_path.up.sql

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+-- Add ebook_folder_path to users table
+ALTER TABLE users ADD COLUMN ebook_folder_path VARCHAR(500);

backend/migrations/005_create_user_ebook_folders.up.sql

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+-- Create user_ebook_folders table for multiple folders per user
+CREATE TABLE user_ebook_folders (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    folder_path VARCHAR(500) NOT NULL,
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    UNIQUE(user_id, folder_path)
+);
+
+-- Create index for faster lookups
+CREATE INDEX idx_user_ebook_folders_user_id ON user_ebook_folders(user_id);

backend/migrations/006_drop_ebook_folder_path.up.sql

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+-- Drop the old ebook_folder_path column from users table
+ALTER TABLE users DROP COLUMN IF EXISTS ebook_folder_path;