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>

docs/core/alerting.md

@@ -14,7 +14,7 @@ When any monitor or plugin calls `record_metric()`:
 4. If a state transition occurs, an `AlertEvent` row is written
 5. Notification I/O is deferred outside the transaction via `asyncio.create_task()`
 
-**Key function:** `roundtable/core/alerts.py` → `record_metric(session, source_module, resource_name, metric_name, value)`
+**Key function:** `steward/core/alerts.py` → `record_metric(session, source_module, resource_name, metric_name, value)`
 
 `record_metric()` must always be called inside an active transaction:
 
@@ -130,7 +130,7 @@ Webhook is skipped if `webhook.url` is empty.
 
 ## Data Models
 
-Defined in `roundtable/models/alerts.py`:
+Defined in `steward/models/alerts.py`:
 
 - **`alert_rules`** — one row per configured rule
 - **`alert_states`** — one row per rule, tracks current state and consecutive failure count