Implement host-based routing with global routing table

Add routing table (HashMap<String, SiteConfig>) to DynamicConfig for
O(1) host lookup. Implement normalize_host (lowercase + strip port)
per RFC 7230 §2.7.3. Add proxy_handler that routes /health to 200,
missing Host to 400, unknown host to 404, and known host to 200.
Routing table updates atomically via ArcSwap.

src/proxy/mod.rs

@@ -1,3 +1,5 @@
 pub mod error;
 pub mod handler;
 pub mod headers;
+
+pub use crate::config::dynamic_config::normalize_host;