fix: index the chat queries that make large SQLite instances unusable (#27663)

The timer scheduler polls once a second and cancels on every message send and chat open, the sidebar lists chats ordered by `updated_at`, and the folder badges count unread chats per folder. None of those could be served by an index, so each call read most of the `chat` table, and because `meta` sits after the chat payload column SQLite had to walk every row's overflow pages to get there. On a large history that stalls the sidebar, every chat switch and every send, and the idle poll alone burns about a quarter of a CPU core.

Timers now keep their due time in a dedicated `chat.timer_at` column behind a partial index, and the chat list, unread and unfinished-reply queries each get an index matching their filter and ordering. Existing pending timers are backfilled from their meta by the migration. Dropping the `internal` and `type` checks also makes a forked timer chat inert, where a fork used to copy `meta` verbatim and become a second claim target that could fire a duplicate timer.

Measured on SQLite, same rows returned:

| query | before | after |
|---|---|---|
| idle timer poll (2000 chats, 0.43 GB) | 170 ms | 0.04 ms |
| cancel on send and chat open (4000 chats, 377 MB) | 200 ms | 0.04 ms |
| sidebar chat list (15000 chats, 1.26 GB) | 157 ms | 1.8 ms |
| folder unread badges (15000 chats, 1.4 GB) | 54 ms | 0.2 ms |

PostgreSQL 17 serves all of them as index-only scans with no sort node. Exercised through fresh install, upgrade with seeded data, downgrade and re-upgrade on SQLite and PostgreSQL 17.

Fixes #27622

backend/open_webui/migrations/versions/d4c1a8e37b62_add_chat_timer_at_and_chat_indexes.py

@@ -0,0 +1,70 @@
+"""add chat timer_at and chat list, unread and timer indexes
+
+Revision ID: d4c1a8e37b62
+Revises: 6d09d1bf1f23
+Create Date: 2026-08-23 18:05:12.441907
+
+"""
+
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = 'd4c1a8e37b62'
+down_revision: str | None = '6d09d1bf1f23'
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.add_column('chat', sa.Column('timer_at', sa.BigInteger(), nullable=True))
+    op.create_index(
+        'timer_at_idx',
+        'chat',
+        ['timer_at'],
+        sqlite_where=sa.text('timer_at IS NOT NULL'),
+        postgresql_where=sa.text('timer_at IS NOT NULL'),
+    )
+
+    # Timers created before this migration carry their due time in meta only, and would never fire.
+    chat = sa.table(
+        'chat', sa.column('id', sa.String), sa.column('meta', sa.JSON), sa.column('timer_at', sa.BigInteger)
+    )
+    conn = op.get_bind()
+    pending = conn.execute(
+        sa.select(chat.c.id, chat.c.meta)
+        .where(chat.c.meta['type'].as_string() == 'timer')
+        .where(chat.c.meta['status'].as_string() == 'pending')
+    ).all()
+    for chat_id, meta in pending:
+        try:  # imported chats can carry any meta, and a non-numeric due time must not abort the migration
+            due_at = int(meta.get('timer_at'))
+        except (TypeError, ValueError):
+            continue
+        conn.execute(chat.update().where(chat.c.id == chat_id).values(timer_at=due_at))
+
+    op.create_index('user_id_updated_at_id_idx', 'chat', ['user_id', sa.text('updated_at DESC'), 'id'])
+    op.create_index(
+        'user_id_timer_at_idx',
+        'chat',
+        ['user_id', 'timer_at'],
+        sqlite_where=sa.text('timer_at IS NOT NULL'),
+        postgresql_where=sa.text('timer_at IS NOT NULL'),
+    )
+    op.create_index(
+        'user_id_folder_unread_idx',
+        'chat',
+        ['user_id', 'folder_id', 'archived', 'updated_at', 'last_read_at', 'id'],
+    )
+    op.create_index('chat_message_chat_role_done_idx', 'chat_message', ['chat_id', 'role', 'done'])
+
+
+def downgrade() -> None:
+    op.drop_index('chat_message_chat_role_done_idx', table_name='chat_message')
+    op.drop_index('user_id_folder_unread_idx', table_name='chat')
+    op.drop_index('user_id_timer_at_idx', table_name='chat')
+    op.drop_index('user_id_updated_at_id_idx', table_name='chat')
+    op.drop_index('timer_at_idx', table_name='chat')
+    op.drop_column('chat', 'timer_at')