fix(config): accept documented compose host

[volschin]

Summary

• accept the documented Compose-only DGX_HOST_IP key as a validated IPv4/IPv6 compatibility field while preserving strict rejection of every other unknown setting
• exclude the compatibility field from settings representations and serialization
• align TODO.md with the stable named Qwen Base clone path and distinguish live-proven production behavior from diagnostic fixture assumptions
• include the approved design and implementation plan

Verification

• TDD red: both documented IPv4/IPv6 cases failed with extra_forbidden before the implementation
• TDD green: valid IPv4/IPv6 accepted; invalid IP and unrelated unknown dotenv key rejected
• real private repository .env loaded as IPv4Address; compatibility field absent from repr and model_dump
• venv/bin/pytest -q — 310 passed
• ruff check agent tests dgx/tts — passed
• ruff format --check agent tests dgx/tts — 53 files already formatted
• all three Compose configurations validated
• git diff --check origin/main...HEAD — passed

Operational cleanup

The already-merged feat/shared-ai-traefik worktree plus its local and remote branches were removed after proving its head was clean and an ancestor of main. The unrelated locked Claude worktree was left untouched.

Live validation

No new runtime deployment is required for this local configuration-boundary fix. The existing Portainer TTS deployment was not changed.