Static serving: correct cache headers, no index fallback for assets

- /assets/*: immutable one-year cache on hits, plain 404 on misses
  (previously a missing asset fell back to index.html served as its
  content type, breaking CSS/JS after every deploy for cached clients)
- index.html and SPA routes: no-cache, so deploys are visible immediately

Cargo.lock

@@ -2574,6 +2574,7 @@ dependencies = [
  "time",
  "tokio",
  "tokio-util",
+ "tower",
  "tower-http",
  "tracing",
  "tracing-subscriber",

Cargo.toml

@@ -37,7 +37,8 @@ tempfile = "3"
 time = "0.3"
 tokio = { version = "1", features = ["full"] }
 tokio-util = { version = "0.7", features = ["io"] }
-tower-http = { version = "0.6", features = ["fs", "trace"] }
+tower = { version = "0.5", features = ["util"] }
+tower-http = { version = "0.6", features = ["fs", "set-header", "trace"] }
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 urlencoding = "2"

src/bin/server.rs

@@ -1,4 +1,8 @@
+use axum::http::{header, HeaderValue};
+use tower::ServiceBuilder;
+use tower_http::services::fs::ServeFileSystemResponseBody;
 use tower_http::services::{ServeDir, ServeFile};
+use tower_http::set_header::SetResponseHeaderLayer;
 use tower_http::trace::TraceLayer;
 use tracing_subscriber::EnvFilter;
 
@@ -17,11 +21,30 @@ async fn main() -> anyhow::Result<()> {
     let state = AppState::new(config).await?;
 
     let static_dir = state.config.static_dir.clone();
-    let index = std::path::Path::new(&static_dir).join("index.html");
-    // .fallback (not .not_found_service) so SPA routes get index.html with a 200
-    let spa = ServeDir::new(&static_dir).fallback(ServeFile::new(index));
+    let static_root = std::path::Path::new(&static_dir);
+    // Hashed assets cache forever — except 404s, which would outlive the next deploy.
+    let assets = ServiceBuilder::new()
+        .map_response(|mut response: axum::http::Response<ServeFileSystemResponseBody>| {
+            if response.status().is_success() {
+                response.headers_mut().insert(
+                    header::CACHE_CONTROL,
+                    HeaderValue::from_static("public, max-age=31536000, immutable"),
+                );
+            }
+            response
+        })
+        .service(ServeDir::new(static_root.join("assets")));
+    // index.html revalidates every load (it names the asset hashes);
+    // .fallback (not .not_found_service) so SPA routes get it with a 200.
+    let spa = ServiceBuilder::new()
+        .layer(SetResponseHeaderLayer::overriding(
+            header::CACHE_CONTROL,
+            HeaderValue::from_static("no-cache"),
+        ))
+        .service(ServeDir::new(static_root).fallback(ServeFile::new(static_root.join("index.html"))));
 
     let app = photos::routes::router(&state)
+        .nest_service("/assets", assets)
         .fallback_service(spa)
         .layer(TraceLayer::new_for_http())
         .with_state(state.clone());