Log real client IPs via Fly-Client-IP header (#130)

## Summary
- Add `client_ip` field to the Fly.io nginx JSON log format, sourced from `Fly-Client-IP` header
- Extract `client_ip` in the Alloy pipeline so it's available as a parsed field in Loki
- Keeps `remote_addr` (the internal proxy IP) for debugging

Fixes: Grafana access logs for docs.eblu.me showing 172.16.11.178 for every request instead of real visitor IPs.

## Deployment and Testing
- [ ] Deploy updated fly.io proxy: `fly deploy` from `fly/` directory
- [ ] Verify in Grafana that new log lines include `client_ip` with real IPs
- [ ] Confirm `remote_addr` still shows the proxy IP (preserved for debugging)

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/130

fly/alloy.river

@@ -24,6 +24,7 @@ loki.process "nginx" {
   // Parse the JSON log line
   stage.json {
     expressions = {
+      client_ip            = "client_ip",
       status               = "status",
       method               = "request_method",
       host                 = "http_host",

fly/nginx.conf

@@ -13,6 +13,7 @@ http {
         '{'
             '"time":"$time_iso8601",'
             '"remote_addr":"$remote_addr",'
+            '"client_ip":"$http_fly_client_ip",'
             '"request_method":"$request_method",'
             '"request_uri":"$request_uri",'
             '"status":$status,'