[SILO-1181] feat: support configurable subpath prefix for MCP server (#106)

* feat: support configurable subpath prefix via MCP_PATH_PREFIX

Allows deploying the HTTP server under an ingress subpath (e.g. /mcp) by
mounting the OAuth, header-auth, and SSE transports under the prefix and
passing the prefix into each OAuth provider's base_url so advertised
metadata URLs (issuer, authorize, token, register, resource) resolve to
the externally-visible path.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: fall back to external URL when PLANE_INTERNAL_BASE_URL is empty string

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>