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/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)
 			}
 		})
 	}