Serve a happy-path MCP tuning recommendation

[sebawebber]

Parent

#43

What to build

Expose the first complete happy path for PostgreSQL tuning over MCP. A compatible
client can initialize against the existing API deployment, discover the single
recommend_postgres_configuration tool, submit a complete valid request, and
receive the provenance-aware result from the shared tuning operation.

Acceptance criteria

• The existing API process serves a public Streamable HTTP MCP endpoint at /mcp, outside the REST version namespaces.
• The server is stateless, uses JSON responses, identifies itself as pgconfig, and reuses the application release version.
• Tool discovery exposes only recommend_postgres_configuration in this release.
• The tool is annotated as read-only, deterministic, and idempotent.
• A complete valid request returns the normalized Tuning Request, Tuning Assumptions, warnings, and recommendations keyed by PostgreSQL parameter name.
• The tool declares an output schema, returns structured content, and mirrors the same result as serialized JSON text fallback.
• MCP recommendations omit listen_addresses, while legacy consumers remain unchanged.
• End-to-end tests through Fiber and an official MCP client cover initialization, discovery, and a successful call.

Blocked by

• Establish the provenance-aware tuning boundary #44