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>

Cargo.toml

@@ -28,3 +28,4 @@ argon2 = "0.5"
 tower-sessions = "0.14"
 tower-sessions-sqlx-store = { version = "0.15", features = ["postgres"] }
 rpassword = "7"
+memory-serve = "2.1"