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>

config.example.yaml

@@ -1,9 +1,9 @@
-# Roundtable — Bootstrap Configuration Example
+# Steward — Bootstrap Configuration Example
 #
 # The only REQUIRED setting is the database URL.
 # Set it via env var (recommended for Docker):
 #
-#   ROUNDTABLE_DATABASE_URL=postgresql+asyncpg://user:pass@host/db
+#   STEWARD_DATABASE_URL=postgresql+asyncpg://user:pass@host/db
 #
 # All other settings (SMTP, webhook, ansible, plugins, retention)
 # are stored in the database and managed via the Settings UI at /settings/.
@@ -11,7 +11,7 @@
 # config.yaml is optional. Use it only if you prefer file-based bootstrap.
 
 database:
-  url: "postgresql+asyncpg://roundtable:password@localhost/roundtable"
+  url: "postgresql+asyncpg://steward:password@localhost/steward"
 
 # Optional: override the auto-generated secret key.
 # If not set, a key is auto-generated on first run and saved to /data/secret.key.