fix: audit round 4 — XSS in HTML report export, SSRF webhook guard, body limit

- ToHTML() now html.EscapeString on ticker, generated_at, section name/body, citations (stored/LLM content can't inject script into exported file)
- Discord destination webhook_url restricted to discord.com/discordapp.com hosts (SSRF guard: server POSTs alert cards; no private IPs or arbitrary hosts)
- Router middleware caps POST/PUT/PATCH bodies at 1 MiB (MaxBytesReader)
- TestDestinations updated: non-discord host must 422, uses realistic discord.com webhook URL

backend/internal/api/server.go

@@ -63,6 +63,16 @@ func New(cfg config.Config, db *store.DB, cache *store.Cache, s *sectors.Client)
 func (s *Server) Router() http.Handler {
 	r := chi.NewRouter()
 	r.Use(middleware.Logger, middleware.Recoverer, middleware.Heartbeat("/ping"))
+	// Cap request bodies (1 MiB) so large POSTs cannot exhaust memory.
+	// JSON bodies here are tiny (auth, screen filters, chat prompts).
+	r.Use(func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+			if req.Body != nil && (req.Method == http.MethodPost || req.Method == http.MethodPut || req.Method == http.MethodPatch) {
+				req.Body = http.MaxBytesReader(w, req.Body, 1<<20)
+			}
+			next.ServeHTTP(w, req)
+		})
+	})
 	r.Route("/api", func(r chi.Router) {
 		r.Get("/health", s.Health)
 		r.Get("/auth/start", s.AuthStart)