Autowire use-case DI, map chat-listing state, keyword-only use cases

[lesnik512]

Four wiring simplifications with no change to the HTTP contract. Rationale and
rejected alternatives live in
planning/changes/2026-08-21.02-di-wiring-and-chat-listing.md.

• Autowired DI. ModernDIPlugin(di_container, autowired_groups=[ioc.UseCases])
  replaces the twelve-entry dependencies= dict and its imports in build_app.
  Database and Repositories stay unwired on purpose.
• Chat listing. ChatsTable maps unread_count (query_expression, filled by
  with_expression) and last_message (viewonly relationship, one selectinload),
  so list_for_user returns entities whose attribute names already match the
  schema. ChatListRow, ChatListItem.from_row and the manual WHERE id IN (...)
  preview batch are gone; the endpoint is one Chats.from_models(...).
• Keyword-only use cases. All ten __call__s, e.g.
  create_chat_use_case(actor=request.user, data=data).
• AuthedRequest. Authenticated handlers annotate the request with the alias
  instead of restating litestar.Request[UsersTable, Any, Any].

Notes for review

• Response JSON is unchanged — tests/api/test_chat_listing_api.py needed no edits.
• list_for_user runs populate_existing=True: sessions are expire_on_commit=False,
  so an identity-mapped chat would otherwise keep the unread_count it was first
  loaded with, and a second listing in the same session would return stale numbers.
  Safe only because the query is read-only.
• last_message_id carries no ForeignKey (it would close a cycle with
  messages.chat_id), so the relationship annotates the join column orm.foreign()
  and folds the soft-delete guard into its primaryjoin.
• No DDL, so no migration. alembic check reports the same pre-existing
  ck_chats_chattype diff as main and nothing else.

Verification

• just test — 108 passed, 100% coverage.
• just lint — clean.
• just check-planning — OK.