fix: set Location header on POST 201 and add WithBaseURL option

[q-uint]

resourcePostHandler never set the HTTP Location header on 201 responses, violating RFC 7644 Section 3.3 which states the server SHALL return the resource URI in the Location header.

Additionally, meta.location and Location headers were always relative paths (e.g. "Users/123"). The RFC examples consistently use absolute URIs. Add a WithBaseURL server option that prepends a configurable base URL to all resource locations, enabling RFC-compliant absolute URIs when configured.

Summary

• Set the HTTP Location header on 201 Created responses in
  resourcePostHandler, as required by RFC 7644 Section 3.3
• Extract resource location construction into a shared
  resourceLocation() function used by all handlers (GET, PATCH,
  POST, PUT) and list responses
• Add WithBaseURL server option to produce absolute URIs in both
  meta.location and the Location header (e.g.
  https://example.com/v2/Users/123 instead of Users/123)
• Without WithBaseURL, behavior is unchanged (relative paths)