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/main.py

@@ -18,6 +18,7 @@ from lembas.api import (
     admin_models,
     admin_prompts,
     admin_search,
+    admin_suggestions,
     admin_users,
     audio,
     auth,
@@ -65,6 +66,7 @@ async def lifespan(app: FastAPI) -> AsyncIterator[None]:
         from lembas.services.chat import sweep_temporary
         from lembas.services.files import sweep_orphans
         from lembas.services.library.documents import sweep_unfiled
+        from lembas.services.suggestions import seed_defaults as seed_suggestions
 
         with session_scope() as db:
             sweep_orphans(db)
@@ -74,6 +76,8 @@ async def lifespan(app: FastAPI) -> AsyncIterator[None]:
             # Temporary chats older than a day. Startup only, like the sweeps
             # above it -- see services/chat.py:sweep_temporary.
             sweep_temporary(db)
+            # Three starting points on the empty screen, written once ever.
+            seed_suggestions(db)
     except Exception:  # noqa: BLE001 - housekeeping must never block startup
         log.exception("orphaned upload sweep failed")
 
@@ -115,6 +119,7 @@ def create_app() -> FastAPI:
     app.include_router(admin_audio.router)
     app.include_router(admin_search.router)
     app.include_router(admin_prompts.router)
+    app.include_router(admin_suggestions.router)
 
     register_error_handlers(app)
     return app