feat(cache): add pingora-cache integration with per-route config

Add in-memory HTTP response cache using pingora-cache MemCache backend.
Cache runs after the detection pipeline so cache hits bypass upstream
request modifications and body rewriting. Respects Cache-Control
(no-store, private, s-maxage, max-age), skips caching for routes with
body rewrites or auth subrequest headers, and supports configurable
default TTL, stale-while-revalidate, and max file size per route.

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

src/metrics.rs

@@ -72,6 +72,19 @@ pub static RATE_LIMIT_DECISIONS: LazyLock<IntCounterVec> = LazyLock::new(|| {
     c
 });
 
+pub static CACHE_STATUS: LazyLock<IntCounterVec> = LazyLock::new(|| {
+    let c = IntCounterVec::new(
+        Opts::new(
+            "sunbeam_cache_status_total",
+            "Cache hit/miss counts",
+        ),
+        &["status"],
+    )
+    .unwrap();
+    REGISTRY.register(Box::new(c.clone())).unwrap();
+    c
+});
+
 pub static ACTIVE_CONNECTIONS: LazyLock<Gauge> = LazyLock::new(|| {
     let g = Gauge::new(
         "sunbeam_active_connections",