Server: harden VeryFS.Server deployment and public API contract

[FerdinandSu]

Scope

Track VeryFS.Server work needed to make the ASP.NET Core host the reference runtime for VeryFS.

Checklist

• Keep VeryFS.Server as the reference ASP.NET Core host for the Core library.
• Harden Docker Compose deployment with PostgreSQL, health checks, and secrets.
• Provide OpenAPI descriptions for mount management, filesystem operations, permissions, and workspace export.
• Add operational guides for reverse proxies, TLS termination, logging, and production security modes.

Acceptance Criteria

• docker compose up --build works from the umbrella repository layout.
• Deployment docs cover PostgreSQL, secrets, health checks, and reverse proxy assumptions.
• OpenAPI output is available and matches the implemented endpoints.
• Production hardening guidance is explicit about auth modes and secret handling.

Source

Roadmap section: docs/roadmap.md / Server.