feat(chat): add presence metadata and improve web chat core

- add user last_seen_at with alembic migration and persist on realtime disconnect
- extend chat serialization with private online/last_seen, group members/online, channel subscribers
- add Redis batch presence lookup helper
- update web chat list/header to display status counters and last-seen labels
- improve delivery receipt handling using last_delivered/last_read boundaries
- include chat info panel and related API/type updates

app/users/models.py

@@ -30,6 +30,7 @@ class User(Base):
         onupdate=func.now(),
         nullable=False,
     )
+    last_seen_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True, index=True)
 
     memberships: Mapped[list["ChatMember"]] = relationship(back_populates="user", cascade="all, delete-orphan")
     sent_messages: Mapped[list["Message"]] = relationship(back_populates="sender")