fix(core): forward visitor IP to Umami and Plausible via X-Forwarded-For

[deer]

• formatUmami was not forwarding the visitor IP at all — Umami was geo-locating events using the Deno Deploy data center IP instead of the real visitor's IP, producing bogus country data
• Moved IP forwarding for Umami from the JSON payload body (which Umami ignores) to the X-Forwarded-For request header, matching how Plausible already worked
• Centralized IP extraction into the middleware: x-forwarded-for from the incoming request (set by upstream proxies) with fallback to ctx.info.remoteAddr (the real client IP on Deno Deploy for direct browser requests)
• Plausible and custom providers updated to receive the same centrally-resolved IP rather than each re-reading headers independently
• Added tests covering the x-forwarded-for → remoteAddr fallback chain and asserting IP appears in outgoing headers (not the payload body) for Umami