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

alembic/versions/0008_user_last_seen_presence.py

@@ -0,0 +1,27 @@
+"""add users.last_seen_at for presence metadata
+
+Revision ID: 0008_user_last_seen_presence
+Revises: 0007_message_hidden_table
+Create Date: 2026-03-08 12:10:00.000000
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+revision: str = "0008_user_last_seen_presence"
+down_revision: Union[str, Sequence[str], None] = "0007_message_hidden_table"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("users", sa.Column("last_seen_at", sa.DateTime(timezone=True), nullable=True))
+    op.create_index(op.f("ix_users_last_seen_at"), "users", ["last_seen_at"], unique=False)
+
+
+def downgrade() -> None:
+    op.drop_index(op.f("ix_users_last_seen_at"), table_name="users")
+    op.drop_column("users", "last_seen_at")