feat(server): embed SPA via memory-serve behind embed-web feature

Adds `memory-serve` 2.1 as an optional workspace dependency, a `build.rs`
that runs `load_directory` only when `CARGO_FEATURE_EMBED_WEB` is set, a
`web_assets` module serving `web/dist` at `/` with SPA fallback (200 OK)
for unknown client-side routes, and a feature-gated integration test.

The default build (no feature) compiles and tests cleanly without `web/dist`.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/server/src/web_assets.rs

@@ -0,0 +1,15 @@
+//! Serves the embedded SPA (built `web/dist`) at `/` with a client-side-routing
+//! fallback. Compiled only with the `embed-web` feature; in dev the SPA is served by
+//! Vite (which proxies `/api` to this server), so this module is absent.
+
+use axum::{Router, http::StatusCode};
+
+/// A router that serves the embedded `web/dist` assets, falling back to `index.html`
+/// for unknown paths so the SPA can own client-side routes.
+pub(crate) fn routes() -> Router {
+    memory_serve::load!()
+        .index_file(Some("/index.html"))
+        .fallback(Some("/index.html"))
+        .fallback_status(StatusCode::OK)
+        .into_router()
+}