Convert backend from submodule to regular directory

Remove submodule tracking; backend is now a plain directory in the repo.
Also update deploy workflow: remove --recurse-submodules.

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

backend/internal/api/spa.go

@@ -0,0 +1,53 @@
+package api
+
+import (
+	"embed"
+	"io/fs"
+	"net/http"
+	"path/filepath"
+	"strings"
+)
+
+// spaFS holds the built Vite SPA.
+// The directory backend/internal/api/webapp/ is populated by the Docker
+// multi-stage build (node → copy dist → go build).
+// A placeholder file keeps the embed valid when building without Docker.
+
+//go:embed webapp
+var spaFS embed.FS
+
+// SPAHandler serves the Vite SPA under the given prefix (e.g. "/app").
+// Static assets (paths with file extensions) are served directly.
+// All other paths fall back to index.html for client-side routing.
+func SPAHandler(prefix string) http.Handler {
+	sub, err := fs.Sub(spaFS, "webapp")
+	if err != nil {
+		return http.NotFoundHandler()
+	}
+	fileServer := http.FileServer(http.FS(sub))
+
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		// Strip the mount prefix to get the file path
+		path := strings.TrimPrefix(r.URL.Path, prefix)
+		if path == "" || path == "/" {
+			// Serve index.html
+			r2 := r.Clone(r.Context())
+			r2.URL.Path = "/index.html"
+			fileServer.ServeHTTP(w, r2)
+			return
+		}
+
+		// Has a file extension → serve asset directly (JS, CSS, fonts, …)
+		if filepath.Ext(path) != "" {
+			r2 := r.Clone(r.Context())
+			r2.URL.Path = path
+			fileServer.ServeHTTP(w, r2)
+			return
+		}
+
+		// SPA route → serve index.html
+		r2 := r.Clone(r.Context())
+		r2.URL.Path = "/index.html"
+		fileServer.ServeHTTP(w, r2)
+	})
+}