fix: critical timestamp parsing and audit fixes

- Fix timestamp conversion: String(epochMs) → new Date(epochMs).toISOString()
  in use-messages-sync, use-comments-sync, use-documents, use-inbox.
  Without this, date comparisons (isEdited, cutoff filters) would fail.
- Fix updated_at: undefined → null in use-inbox to match InboxItem type
- Fix ZeroProvider: skip Zero connection for unauthenticated users
- Clean 30+ stale "Electric SQL" comments in backend Python code

surfsense_backend/app/db.py

@@ -712,7 +712,7 @@ class ChatComment(BaseModel, TimestampMixin):
         nullable=False,
         index=True,
     )
-    # Denormalized thread_id for efficient Electric SQL subscriptions (one per thread)
+    # Denormalized thread_id for efficient Zero subscriptions (one per thread)
     thread_id = Column(
         Integer,
         ForeignKey("new_chat_threads.id", ondelete="CASCADE"),
@@ -782,7 +782,7 @@ class ChatCommentMention(BaseModel, TimestampMixin):
 class ChatSessionState(BaseModel):
     """
     Tracks real-time session state for shared chat collaboration.
-    One record per thread, synced via Electric SQL.
+    One record per thread, synced via Zero.
     """
 
     __tablename__ = "chat_session_state"