perf(cache): use Arc<str> for etag/content_type in ContentCachePort

Replace String with Arc<str> for etag and content_type fields in the
content cache. String::clone() allocates and copies the full string on
every cache hit (O(n)), while Arc<str>::clone() is O(1) — just an atomic
ref-count increment.

This eliminates 2 heap allocations per cache hit on the hottest download
path. At 1000 req/s that is 2000 fewer alloc/dealloc cycles per second.

Changed files:
- cache_ports.rs: trait signatures String → Arc<str>
- file_content_cache.rs: CacheEntry fields, get/put methods, tests
- stubs.rs: StubContentCachePort signatures
- file_retrieval_service.rs: caller creates Arc<str> before put()

src/application/services/file_retrieval_service.rs

@@ -197,13 +197,14 @@ impl FileRetrievalService {
 
             // Store in cache
             if let Some(cache) = &self.content_cache {
-                let etag = format!("\"{}-{}\"", id, modified_at);
+                let etag: Arc<str> = format!("\"{}-{}\"", id, modified_at).into();
+                let ct: Arc<str> = Arc::from(&*mime_type);
                 cache
                     .put(
                         id.to_string(),
                         content_bytes.clone(),
                         etag,
-                        mime_type.clone(),
+                        ct,
                     )
                     .await;
             }