chore: rename project Roundtable → Steward

Renames the Python package directory, CLI command, env var prefix,
docker-compose service/container/image, Postgres role/db, and all
visible branding. Marketing form is "Fabled Steward".

Clean break from the previous rebrand: drops the fabledscryer→roundtable
import shim in __init__.py and the FABLEDSCRYER_* env var fallback in
config.py and migrations/env.py. Env vars are now STEWARD_* only.

Heads-up for existing deployments:
- Postgres user/db renamed fabledscryer → steward in docker-compose.yml.
  Existing volumes need the role/db renamed inside Postgres, or override
  POSTGRES_USER/POSTGRES_DB to keep the old names.
- Host-agent systemd unit is now steward-agent.service. Existing agents
  keep running under the old name; reinstall to switch.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

steward/database.py

@@ -0,0 +1,20 @@
+from __future__ import annotations
+from typing import TYPE_CHECKING
+from sqlalchemy.ext.asyncio import create_async_engine, async_sessionmaker, AsyncSession
+
+if TYPE_CHECKING:
+    from quart import Quart
+
+
+def init_db(app: "Quart") -> None:
+    """Create async engine and attach db_sessionmaker to app.
+
+    Called during create_app() after config is loaded.
+    Does not create tables — Alembic handles migrations.
+    """
+    db_url: str = app.config["DATABASE_URL"]
+    engine = create_async_engine(db_url, echo=False)
+    app.db_sessionmaker: async_sessionmaker[AsyncSession] = async_sessionmaker(
+        engine, expire_on_commit=False
+    )
+    app._db_engine = engine