feat: Implement document permission handling and sharing features

backend/scripts/004_add_share_link_permission.sql

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+-- Migration: Add permission column for public share links
+-- Dependencies: Run after 003_add_public_sharing.sql
+-- Purpose: Store permission level (view/edit) for public share links
+
+-- Add permission column to documents table
+ALTER TABLE documents ADD COLUMN IF NOT EXISTS share_permission VARCHAR(20) DEFAULT 'edit' CHECK (share_permission IN ('view', 'edit'));
+
+-- Create index for performance
+CREATE INDEX IF NOT EXISTS idx_documents_share_permission ON documents(share_permission) WHERE is_public = true;
+
+-- Documentation
+COMMENT ON COLUMN documents.share_permission IS 'Permission level for public share link: view (read-only) or edit (read-write). Defaults to edit for backward compatibility.';