chore(gateway): drop schweitz.internal, default to http://tatlock:8000

The homelab is retiring the *.schweitz.internal domain; in-network
machine-to-machine traffic uses docker container names on the
docker-dataplane network. The deployed tatlock-ui stack already overrides
DESKLOCK_TATLOCK_BASE_URL (Tatlock runs on the host), so only the
fallback default changes.

Docs follow: AGENTS.md M2M guidance now points at container names with
*.schweitz.net reserved for browsers, architecture.md drops the retired
domain (the registry name now matches what CI actually pushes since
c477019), and the README diagram loses the stale hostname.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

gateway/src/desklock_gateway/config.py

@@ -4,7 +4,7 @@ from pydantic_settings import BaseSettings
 class Settings(BaseSettings):
     """Gateway configuration, overridable via DESKLOCK_* environment variables."""
 
-    tatlock_base_url: str = "http://tatlock.schweitz.internal:8000"
+    tatlock_base_url: str = "http://tatlock:8000"
     tatlock_model: str = "Tatlock"
 
     # PCM rate of the device WebSocket contract (docs/architecture.md)