feat(server): security headers and a hash-based CSP for the web app (M462 #4980)

There were no security headers at all. Now:
- every response: nosniff, Referrer-Policy strict-origin-when-cross-origin,
  Permissions-Policy (no camera/mic/geolocation), X-Frame-Options DENY;
  each set only if the handler hasn't.
- HSTS only when the trusted proxy reports HTTPS (rule 94); never a redirect.
- index.html carries a Content-Security-Policy whose script-src is 'self'
  plus the sha256 of each inline script in the page as served, computed
  after the branding template runs. No 'unsafe-inline' or 'unsafe-eval'
  for scripts. img-src admits remote https/http because Lidarr suggestion
  art is a remote poster URL.

Hashing in Go rather than via SvelteKit's kit.csp covers the inline scripts
SvelteKit doesn't know about (app.html's theme bootstrap and the branding
global injected at build) and stays correct whatever the app name is.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/embed.go

@@ -47,12 +47,14 @@ func Handler(branding config.BrandingConfig) http.Handler {
 		panic("web: branding template failed: " + err.Error())
 	}
 
+	csp := contentSecurityPolicy(index)
+
 	fileServer := http.FileServer(http.FS(sub))
 
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		clean := path.Clean(r.URL.Path)
 		if clean == "/" || clean == "." {
-			serveIndex(w, index)
+			serveIndex(w, index, csp)
 			return
 		}
 		name := strings.TrimPrefix(clean, "/")
@@ -60,12 +62,16 @@ func Handler(branding config.BrandingConfig) http.Handler {
 			fileServer.ServeHTTP(w, r)
 			return
 		}
-		serveIndex(w, index)
+		serveIndex(w, index, csp)
 	})
 }
 
-func serveIndex(w http.ResponseWriter, index []byte) {
+func serveIndex(w http.ResponseWriter, index []byte, csp string) {
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	// The policy only means anything on the document; JSON, audio and image
+	// responses can't run script, so it rides here rather than on every
+	// response.
+	w.Header().Set("Content-Security-Policy", csp)
 	w.Header().Set("Cache-Control", "no-cache")
 	_, _ = w.Write(index)
 }