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/monitors.md

@@ -1,13 +1,13 @@
 # Core Monitors
 
-Roundtable ships two built-in monitors: Ping and DNS. Both run as asyncio scheduled tasks on the same event loop as the web server, with no separate processes.
+Steward ships two built-in monitors: Ping and DNS. Both run as asyncio scheduled tasks on the same event loop as the web server, with no separate processes.
 
 ---
 
 ## Ping Monitor
 
-**Source:** `roundtable/monitors/ping.py`
-**Scheduler task:** `ping_monitor` in `roundtable/app.py`
+**Source:** `steward/monitors/ping.py`
+**Scheduler task:** `ping_monitor` in `steward/app.py`
 **Interval:** `monitors.poll_interval_seconds` (default 60s), runs on startup
 
 ### How It Works
@@ -29,7 +29,7 @@ Each probe writes a `PingResult` row and calls `record_metric()`:
 
 ### Data Model
 
-`ping_results` table (defined in `roundtable/models/monitors.py`):
+`ping_results` table (defined in `steward/models/monitors.py`):
 
 | Column | Type | Description |
 |---|---|---|
@@ -51,8 +51,8 @@ Old rows are pruned by the `data_cleanup` task (default: 90 days).
 
 ## DNS Monitor
 
-**Source:** `roundtable/monitors/dns.py`
-**Scheduler task:** `dns_monitor` in `roundtable/app.py`
+**Source:** `steward/monitors/dns.py`
+**Scheduler task:** `dns_monitor` in `steward/app.py`
 **Interval:** `monitors.poll_interval_seconds` (default 60s), runs on startup
 
 ### How It Works
@@ -70,7 +70,7 @@ Each check writes a `DnsResult` row and calls `record_metric()`:
 
 ### Data Model
 
-`dns_results` table (defined in `roundtable/models/monitors.py`):
+`dns_results` table (defined in `steward/models/monitors.py`):
 
 | Column | Type | Description |
 |---|---|---|