fix: address review findings for observability PR

1. Security: Replace insecure extractRemoteIP() in audit service with
   middleware.RealIP() which validates trusted proxies before trusting
   X-Real-IP/X-Forwarded-For headers. Export RealIP from middleware.
   Update audit tests to verify anti-spoofing behavior.

2. Audit coverage: Add audit instrumentation to all 9 handlers that
   had dead action constants: HandleEnvVarSave, HandleLabelAdd,
   HandleLabelEdit, HandleLabelDelete, HandleVolumeAdd, HandleVolumeEdit,
   HandleVolumeDelete, HandlePortAdd, HandlePortDelete.

3. README: Fix API path from /api/audit to /api/v1/audit.

4. README: Fix duplicate numbering in DI order section (items 10-11
   were listed twice, now correctly numbered 10-16).

internal/middleware/middleware.go

@@ -105,7 +105,7 @@ func (m *Middleware) Logging() func(http.Handler) http.Handler {
 					"request_id", reqID,
 					"referer", request.Referer(),
 					"proto", request.Proto,
-					"remoteIP", realIP(request),
+					"remoteIP", RealIP(request),
 					"status", lrw.statusCode,
 					"bytes", lrw.bytesWritten,
 					"latency_ms", latency.Milliseconds(),
@@ -170,11 +170,11 @@ func isTrustedProxy(ip net.IP) bool {
 	return false
 }
 
-// realIP extracts the client's real IP address from the request.
+// RealIP extracts the client's real IP address from the request.
 // Proxy headers (X-Real-IP, X-Forwarded-For) are only trusted when the
 // direct connection originates from an RFC1918/loopback address.
 // Otherwise, headers are ignored and RemoteAddr is used (fail closed).
-func realIP(r *http.Request) string {
+func RealIP(r *http.Request) string {
 	addr := ipFromHostPort(r.RemoteAddr)
 	remoteIP := net.ParseIP(addr)
 
@@ -365,7 +365,7 @@ func (m *Middleware) LoginRateLimit() func(http.Handler) http.Handler {
 			writer http.ResponseWriter,
 			request *http.Request,
 		) {
-			ip := realIP(request)
+			ip := RealIP(request)
 			limiter := loginLimiter.getLimiter(ip)
 
 			if !limiter.Allow() {

internal/middleware/realip_test.go

@@ -1,4 +1,4 @@
-package middleware //nolint:testpackage // tests unexported realIP function
+package middleware //nolint:testpackage // tests RealIP via internal package access
 
 import (
 	"context"
@@ -126,9 +126,9 @@ func TestRealIP(t *testing.T) { //nolint:funlen // table-driven test
 				req.Header.Set("X-Forwarded-For", tt.xff)
 			}
 
-			got := realIP(req)
+			got := RealIP(req)
 			if got != tt.want {
-				t.Errorf("realIP() = %q, want %q", got, tt.want)
+				t.Errorf("RealIP() = %q, want %q", got, tt.want)
 			}
 		})
 	}