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/models/share.go

@@ -0,0 +1,30 @@
+package models
+
+import (
+	"time"
+
+	"github.com/google/uuid"
+)
+
+type DocumentShare struct {
+    ID         uuid.UUID  `json:"id"`
+    DocumentID uuid.UUID  `json:"document_id"`
+    UserID     uuid.UUID  `json:"user_id"`
+    Permission string     `json:"permission"` // "view" or "edit"
+    CreatedAt  time.Time  `json:"created_at"`
+    CreatedBy  *uuid.UUID `json:"created_by"`
+}
+
+type CreateShareRequest struct {
+    UserEmail  string `json:"user_email" binding:"required"`
+    Permission string `json:"permission" binding:"required,oneof=view edit"`
+}
+
+type ShareListResponse struct {
+    Shares []DocumentShareWithUser `json:"shares"`
+}
+
+type DocumentShareWithUser struct {
+    DocumentShare
+    User User `json:"user"`
+}