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>

README.md

@@ -38,8 +38,8 @@ happens on this server.
                                      ▼       │  Speaches  (container, GPU) │
                      ┌────────────────────┐  │  • STT: faster-whisper      │
                      │  Tatlock (butler)  │  │  • TTS: Kokoro / Piper      │
-                     │  tatlock.schweitz. │  │  also usable by Open WebUI, │
-                     │  internal :8000    │  │  Home Assistant, …          │
+                     │  http://tatlock    │  │  also usable by Open WebUI, │
+                     │  :8000             │  │  Home Assistant, …          │
                      └────────────────────┘  └─────────────────────────────┘
 ```