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/001_add_users_and_sessions.sql

@@ -0,0 +1,52 @@
+-- Migration: Add users and sessions tables for authentication
+-- Run this before 002_add_document_shares.sql
+
+-- Enable UUID extension
+CREATE EXTENSION IF NOT EXISTS "uuid-ossp";
+
+-- Users table
+CREATE TABLE IF NOT EXISTS users (
+    id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+    email VARCHAR(255) NOT NULL,
+    name VARCHAR(255) NOT NULL,
+    avatar_url TEXT,
+    provider VARCHAR(50) NOT NULL CHECK (provider IN ('google', 'github')),
+    provider_user_id VARCHAR(255) NOT NULL,
+    created_at TIMESTAMPTZ DEFAULT NOW(),
+    updated_at TIMESTAMPTZ DEFAULT NOW(),
+    last_login_at TIMESTAMPTZ,
+    UNIQUE(provider, provider_user_id)
+);
+
+CREATE INDEX idx_users_email ON users(email);
+CREATE INDEX idx_users_provider ON users(provider, provider_user_id);
+
+COMMENT ON TABLE users IS 'Stores user accounts from OAuth providers';
+COMMENT ON COLUMN users.provider IS 'OAuth provider: google or github';
+COMMENT ON COLUMN users.provider_user_id IS 'User ID from OAuth provider';
+
+-- Sessions table
+CREATE TABLE IF NOT EXISTS sessions (
+    id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    token_hash VARCHAR(64) NOT NULL,
+    expires_at TIMESTAMPTZ NOT NULL,
+    created_at TIMESTAMPTZ DEFAULT NOW(),
+    user_agent TEXT,
+    ip_address VARCHAR(45),
+    UNIQUE(token_hash)
+);
+
+CREATE INDEX idx_sessions_user_id ON sessions(user_id);
+CREATE INDEX idx_sessions_token_hash ON sessions(token_hash);
+CREATE INDEX idx_sessions_expires_at ON sessions(expires_at);
+
+COMMENT ON TABLE sessions IS 'Stores active JWT sessions for revocation support';
+COMMENT ON COLUMN sessions.token_hash IS 'SHA-256 hash of JWT token';
+COMMENT ON COLUMN sessions.user_agent IS 'User agent string for device tracking';
+
+-- Add owner_id to documents table if it doesn't exist
+ALTER TABLE documents ADD COLUMN IF NOT EXISTS owner_id UUID REFERENCES users(id) ON DELETE SET NULL;
+CREATE INDEX IF NOT EXISTS idx_documents_owner_id ON documents(owner_id);
+
+COMMENT ON COLUMN documents.owner_id IS 'User who created the document';

backend/scripts/002_add_document_shares.sql

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+-- 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)';