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/repository.py

@@ -1,4 +1,5 @@
 from sqlalchemy import Select, select
+from sqlalchemy.orm import aliased
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.chats.models import Chat, ChatMember, ChatMemberRole, ChatType
@@ -60,3 +61,21 @@ async def list_user_chat_ids(db: AsyncSession, *, user_id: int) -> list[int]:
         select(ChatMember.chat_id).where(ChatMember.user_id == user_id).order_by(ChatMember.chat_id.asc())
     )
     return list(result.scalars().all())
+
+
+async def find_private_chat_between_users(db: AsyncSession, *, user_a_id: int, user_b_id: int) -> Chat | None:
+    cm_a = aliased(ChatMember)
+    cm_b = aliased(ChatMember)
+    stmt = (
+        select(Chat)
+        .join(cm_a, cm_a.chat_id == Chat.id)
+        .join(cm_b, cm_b.chat_id == Chat.id)
+        .where(
+            Chat.type == ChatType.PRIVATE,
+            cm_a.user_id == user_a_id,
+            cm_b.user_id == user_b_id,
+        )
+        .limit(1)
+    )
+    result = await db.execute(stmt)
+    return result.scalar_one_or_none()

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,