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/Cargo.toml

@@ -11,6 +11,9 @@ path = "src/lib.rs"
 name = "server"
 path = "src/main.rs"
 
+[features]
+embed-web = ["dep:memory-serve"]
+
 [dependencies]
 tokio.workspace = true
 axum.workspace = true
@@ -24,10 +27,16 @@ db = { path = "../db" }
 domain = { path = "../domain" }
 search = { path = "../search" }
 rpassword.workspace = true
+memory-serve = { workspace = true, optional = true }
+
+[build-dependencies]
+memory-serve = { workspace = true }
 
 [dev-dependencies]
 reqwest.workspace = true
 serde_json.workspace = true
+tower.workspace = true
+http-body-util.workspace = true
 api = { path = "../api" }
 auth = { path = "../auth" }
 db = { path = "../db" }