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/config/validation.rs

@@ -335,22 +335,22 @@ mod tests {
     }
 
     fn valid_dynamic_config() -> DynamicConfig {
-        DynamicConfig {
-            sites: vec![SiteConfig {
+        DynamicConfig::from_sites(
+            vec![SiteConfig {
                 host: "test.local".to_string(),
                 upstream: "127.0.0.1:8080".to_string(),
                 upstream_scheme: "http".to_string(),
                 upstream_connect_timeout_secs: 5,
                 upstream_request_timeout_secs: 60,
             }],
-            rate_limit: RateLimitConfig {
+            RateLimitConfig {
                 requests_per_second: 10,
                 burst: 20,
             },
-            body: BodyConfig {
+            BodyConfig {
                 limit_bytes: 104857600,
             },
-        }
+        )
     }
 
     fn make_static_with_sites(sites: Vec<SiteConfig>, tls: TlsConfig) -> StaticConfig {