feat: add Content-Security-Policy middleware

Add CSP header to all HTTP responses for defense-in-depth against XSS.

The policy restricts all resource loading to same-origin and disables
dangerous features (object embeds, framing, base tag injection). The
embedded SPA requires no inline scripts or inline style attributes
(Preact applies styles programmatically via DOM properties), so a
strict policy without 'unsafe-inline' works correctly.

Directives:
  default-src 'self'      — baseline same-origin restriction
  script-src 'self'       — same-origin scripts only
  style-src 'self'        — same-origin stylesheets only
  connect-src 'self'      — same-origin fetch/XHR only
  img-src 'self'          — same-origin images only
  font-src 'self'         — same-origin fonts only
  object-src 'none'       — no plugin content
  frame-ancestors 'none'  — prevent clickjacking
  base-uri 'self'         — prevent base tag injection
  form-action 'self'      — restrict form submissions

internal/middleware/middleware.go

@@ -180,3 +180,36 @@ func (mware *Middleware) MetricsAuth() func(http.Handler) http.Handler {
 		},
 	)
 }
+
+// cspPolicy is the Content-Security-Policy header value applied to all
+// responses.  The embedded SPA loads scripts and styles from same-origin
+// files only (no inline scripts or inline style attributes), so a strict
+// policy works without 'unsafe-inline'.
+const cspPolicy = "default-src 'self'; " +
+	"script-src 'self'; " +
+	"style-src 'self'; " +
+	"connect-src 'self'; " +
+	"img-src 'self'; " +
+	"font-src 'self'; " +
+	"object-src 'none'; " +
+	"frame-ancestors 'none'; " +
+	"base-uri 'self'; " +
+	"form-action 'self'"
+
+// CSP returns middleware that sets the Content-Security-Policy header on
+// every response for defense-in-depth against XSS.
+func (mware *Middleware) CSP() func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(
+			func(
+				writer http.ResponseWriter,
+				request *http.Request,
+			) {
+				writer.Header().Set(
+					"Content-Security-Policy",
+					cspPolicy,
+				)
+				next.ServeHTTP(writer, request)
+			})
+	}
+}

internal/server/routes.go

@@ -29,6 +29,7 @@ func (srv *Server) SetupRoutes() {
 	}
 
 	srv.router.Use(srv.mw.CORS())
+	srv.router.Use(srv.mw.CSP())
 	srv.router.Use(middleware.Timeout(routeTimeout))
 
 	if srv.sentryEnabled {