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/internal/store/user.go

@@ -0,0 +1,82 @@
+package store
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+
+	"github.com/M1ngdaXie/realtime-collab/internal/models"
+	"github.com/google/uuid"
+)
+
+// UpsertUser creates or updates user from OAuth profile
+func (s *PostgresStore) UpsertUser(ctx context.Context, provider, providerUserID, email, name string, avatarURL *string) (*models.User, error) {
+    query := `
+        INSERT INTO users (provider, provider_user_id, email, name, avatar_url, last_login_at)
+        VALUES ($1, $2, $3, $4, $5, NOW())
+        ON CONFLICT (provider, provider_user_id)
+        DO UPDATE SET
+            email = EXCLUDED.email,
+            name = EXCLUDED.name,
+            avatar_url = EXCLUDED.avatar_url,
+            last_login_at = NOW(),
+            updated_at = NOW()
+        RETURNING id, email, name, avatar_url, provider, provider_user_id, created_at, updated_at, last_login_at
+    `
+
+    var user models.User
+    err := s.db.QueryRowContext(ctx, query, provider, providerUserID, email, name, avatarURL).Scan(
+        &user.ID, &user.Email, &user.Name, &user.AvatarURL, &user.Provider,
+        &user.ProviderUserID, &user.CreatedAt, &user.UpdatedAt, &user.LastLoginAt,
+    )
+    if err != nil {
+        return nil, err
+    }
+    fmt.Printf("✅ User Upserted: ID=%s, Email=%s\n", user.ID.String(), user.Email)
+
+    return &user, nil
+}
+
+// GetUserByID retrieves user by ID
+func (s *PostgresStore) GetUserByID(ctx context.Context, userID uuid.UUID) (*models.User, error) {
+    query := `
+        SELECT id, email, name, avatar_url, provider, provider_user_id, created_at, updated_at, last_login_at
+        FROM users WHERE id = $1
+    `
+
+    var user models.User
+    err := s.db.QueryRowContext(ctx, query, userID).Scan(
+        &user.ID, &user.Email, &user.Name, &user.AvatarURL, &user.Provider,
+        &user.ProviderUserID, &user.CreatedAt, &user.UpdatedAt, &user.LastLoginAt,
+    )
+    if err == sql.ErrNoRows {
+        return nil, nil
+    }
+    if err != nil {
+        return nil, err
+    }
+
+    return &user, nil
+}
+
+// GetUserByEmail retrieves user by email
+func (s *PostgresStore) GetUserByEmail(ctx context.Context, email string) (*models.User, error) {
+    query := `
+        SELECT id, email, name, avatar_url, provider, provider_user_id, created_at, updated_at, last_login_at
+        FROM users WHERE email = $1
+    `
+
+    var user models.User
+    err := s.db.QueryRowContext(ctx, query, email).Scan(
+        &user.ID, &user.Email, &user.Name, &user.AvatarURL, &user.Provider,
+        &user.ProviderUserID, &user.CreatedAt, &user.UpdatedAt, &user.LastLoginAt,
+    )
+    if err == sql.ErrNoRows {
+        return nil, nil
+    }
+    if err != nil {
+        return nil, err
+    }
+
+    return &user, nil
+}