feat: Enhance real-time collaboration features with user awareness and document sharing

- Added user information (UserID, UserName, UserAvatar) to Client struct for presence tracking.
- Implemented failure handling in the broadcastMessage function to manage send failures and disconnect clients if necessary.
- Introduced document ownership and sharing capabilities:
  - Added OwnerID and Is_Public fields to Document model.
  - Created DocumentShare model for managing document sharing with permissions.
  - Implemented functions for creating, listing, and managing document shares in the Postgres store.
- Added user management functionality:
  - Created User model and associated functions for user management in the Postgres store.
  - Implemented session management with token hashing for security.
- Updated database schema with migrations for users, sessions, and document shares.
- Enhanced frontend Yjs integration with awareness event logging for user connections and disconnections.

backend/scripts/002_add_document_shares.sql

@@ -0,0 +1,19 @@
+-- Migration: Add document sharing with permissions
+-- Run against existing database
+
+CREATE TABLE IF NOT EXISTS document_shares (
+    id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+    document_id UUID NOT NULL REFERENCES documents(id) ON DELETE CASCADE,
+    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    permission VARCHAR(20) NOT NULL CHECK (permission IN ('view', 'edit')),
+    created_at TIMESTAMPTZ DEFAULT NOW(),
+    created_by UUID REFERENCES users(id) ON DELETE SET NULL,
+    UNIQUE(document_id, user_id)
+);
+
+CREATE INDEX idx_shares_document_id ON document_shares(document_id);
+CREATE INDEX idx_shares_user_id ON document_shares(user_id);
+CREATE INDEX idx_shares_permission ON document_shares(document_id, permission);
+
+COMMENT ON TABLE document_shares IS 'Stores per-user document access permissions';
+COMMENT ON COLUMN document_shares.permission IS 'Access level: view (read-only) or edit (read-write)';