Prompt suggestions on the new-chat screen

A blank composer is the least helpful thing a chat client can show someone
who has just installed one. Three cards now sit under the empty state, and
an administrator manages them at /admin/suggestions.

Clicking a card fills the composer and stops there. It deliberately does not
send: every default ends mid-sentence, because a card is a starting point
rather than a question somebody already asked, and the caret lands where the
person has to start typing.

Seeding is guarded by a settings flag, not by "is the table empty" --
otherwise an administrator who decided against them would get all three back
on every restart. Capped at twelve, six shown: past a dozen this is a menu,
and a menu on the empty screen is a worse blank page than a blank page.

The cards are gated on there being no chat at all, not on the thread being
empty. An empty chat someone opened on purpose already has a model and a
prompt chosen.

Also fixes a pre-existing bug the position test caught. Both this and
_refresh_models wrote `coalesce(max(position), -1) or -1`, and position 0 is
falsy -- so the second row landed back on 0 on top of the first. The
coalesce was already doing that job; the `or` was undoing it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/db/models/__init__.py

@@ -41,6 +41,7 @@ from lembas.db.models.library import (
     chat_knowledge_bases,
 )
 from lembas.db.models.setting import Setting
+from lembas.db.models.suggestion import Suggestion
 from lembas.db.models.user import (
     ROLE_ADMIN,
     ROLE_PENDING,
@@ -85,6 +86,7 @@ __all__ = [
     "Share",
     "Skill",
     "SkillRevision",
+    "Suggestion",
     "User",
     "chat_knowledge_bases",
     "model_groups",