Cap request body size and add baseline security headers

Part of a security-hardening pass (see wiki for the full backlog).
decodeJSON had no size limit at all, so every JSON endpoint -- including
the two unauthenticated ones (bootstrap, the Alertmanager webhook) --
would buffer an attacker-supplied body of unbounded size before it was
even validated. decodeJSON now wraps the body in http.MaxBytesReader at
a 1 MiB default; the webhook gets its own 8 MiB cap via decodeJSONLimit,
since a real Alertmanager batch can be bigger than an ordinary API body.

Also adds a securityHeaders middleware, applied globally: nosniff on
every response (previously only the static site got it), and HSTS
(180-day max-age, conservative on purpose) whenever cookieSecure's
signal says the browser is on HTTPS. Checked the chart/gateway config
first -- neither sets HSTS anywhere, so this was a real gap.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/api/router.go

@@ -30,6 +30,7 @@ func NewRouter(db *sql.DB, notify NotifyConfig, cfg config.Config, version strin
 	r := chi.NewRouter()
 	r.Use(middleware.Logger)
 	r.Use(middleware.Recoverer)
+	r.Use(securityHeaders(notify.PublicURL))
 
 	r.Get("/healthz", func(w http.ResponseWriter, r *http.Request) {
 		respond(w, http.StatusOK, map[string]string{"status": "ok"})