feat: improve chat realtime and media composer UX

- add media preview and upload confirmation for image/video

- add upload progress tracking for presigned uploads

- keep voice recording/upload flow with better UI states

- include related realtime/chat updates currently in working tree

app/chats/service.py

@@ -16,6 +16,14 @@ async def create_chat_for_user(db: AsyncSession, *, creator_id: int, payload: Ch
             status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
             detail="Private chat requires exactly one target user.",
         )
+    if payload.type == ChatType.PRIVATE:
+        existing_chat = await repository.find_private_chat_between_users(
+            db,
+            user_a_id=creator_id,
+            user_b_id=member_ids[0],
+        )
+        if existing_chat:
+            return existing_chat
     if payload.type in {ChatType.GROUP, ChatType.CHANNEL} and not payload.title:
         raise HTTPException(
             status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,