feat(metrics): add Prometheus metrics and scrape endpoint

Add a prometheus metrics module with counters for requests, DDoS/scanner/
rate-limit decisions, active connections gauge, and request duration
histogram. Spawn a lightweight HTTP server on a configurable port
(default 9090) serving /metrics and /health endpoints.

Signed-off-by: Sienna Meridian Satterwhite <sienna@sunbeam.pt>

Cargo.toml

@@ -49,6 +49,9 @@ arc-swap    = "1"
 # Reverse DNS for bot IP verification
 dns-lookup  = "2"
 
+# Prometheus metrics
+prometheus  = "0.13"
+
 # Rustls crypto provider — must be installed before any TLS init
 rustls      = { version = "0.23", features = ["aws-lc-rs"] }