feat(mcp-docugen): scaffold service + Docker stack con gateway Caddy

Task 0 del piano (adattato a workspace uv):
- services/mcp-docugen/: pyproject.toml, src/mcp_docugen/, tests/unit+integration/,
  README, .env.example. Package rinominato da docugen_mcp -> mcp_docugen.
- Root pyproject.toml: aggiunto services/mcp-docugen a workspace members.
- .python-version: 3.11
- uv.lock committato.

Docker stack stile CerberoSuite/Cerbero con prefisso "arca-":
- docker/base.Dockerfile -> arca-base:latest
- docker/mcp-docugen.Dockerfile -> arca-mcp-docugen:dev (porta interna 9100,
  label arca.service, runtime multi-stage, user non-root, healthcheck)
- docker-compose.yml root: gateway Caddy unica porta host (8080) + mcp-docugen
  su rete interna. Security defaults cap_drop ALL, no-new-privileges, read_only
  ove applicabile, restart unless-stopped.
- gateway/Caddyfile: reverse proxy /mcp-docugen/* -> mcp-docugen:9100 + landing.
- gateway/public/index.html: landing page minimale.

.env.example root aggiornato con DOCUGEN_API_KEY + OPENROUTER_API_KEY condivisa.

Task 1-12 (implementazione TDD effettiva) ancora da fare.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

gateway/Caddyfile

@@ -0,0 +1,21 @@
+{
+	admin off
+	auto_https off
+}
+
+:8080 {
+	log {
+		output stdout
+		format console
+	}
+
+	handle_path /mcp-docugen/* {
+		reverse_proxy mcp-docugen:9100
+	}
+
+	# Landing page statica
+	handle {
+		root * /srv
+		file_server
+	}
+}