scaffold: project skeleton, schema, healthz/readyz, CI

Initial project structure for neuronetz-gateway per scope-docs/SPEC.md:

- Python 3.12 / FastAPI / SQLAlchemy 2.0 (async) / Redis / Postgres stack
  managed by uv. Multi-stage non-root Dockerfile, prod + dev compose files
  (ollama service is NEVER published in either), Caddyfile + systemd unit,
  justfile, GitHub Actions CI (ruff, mypy --strict, pytest, bandit, pip-audit).
- Pydantic-Settings config covering every env var from SPEC §7, including the
  MODEL_DISCOVERY_* keys for the dynamic-discovery feature (§4.6).
- Alembic 0001_initial creates the full gateway schema (8 tables, 3 enums,
  notify_key_revoked() trigger), incl. allow_all_models on tenant_limits and
  key_limits for the per-tenant auto-grant toggle.
- Working /healthz, /readyz (fail-closed when deps unreachable), and a
  Prometheus /metrics stub. Sanitizing error handlers that attach X-Request-ID
  to every response and never leak upstream internals.
- SPEC + AGENT_PROMPT included under scope-docs/ (source of truth).

src/neuronetz_gateway/observability/__init__.py

@@ -0,0 +1,3 @@
+"""Observability: structured logging and Prometheus metrics."""
+
+from __future__ import annotations