Harden and observe public MCP execution

[sebawebber]

Parent

#43

What to build

Make anonymous MCP execution safe and observable in the existing public API
deployment without introducing authentication, application-level caching, custom
rate limiting, or a second service.

Acceptance criteria

• Native clients without an Origin header can initialize and call the tool.
• Browser requests from configured official pgconfig origins are accepted and unrelated browser origins are rejected.
• Each tool execution has a five-second timeout and timeout failures are returned as actionable tool errors.
• Successful calls emit structured logs containing tool, status, duration in milliseconds, assumption count, warning count, and server version.
• Failed calls emit structured logs containing status, a stable error code, and missing-field names when applicable.
• Complete tool arguments and full Tuning Requests are not written to logs.
• The endpoint remains public, anonymous, stateless, and subject to the existing Fiber body-size default.
• Tests cover Origin handling, timeout behavior, success logs, error logs, and the absence of complete arguments from logs.

Blocked by

• Complete MCP validation, defaults, and self-correction #46