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/helpers.go

@@ -72,8 +72,25 @@ func respond(w http.ResponseWriter, status int, v any) {
 	json.NewEncoder(w).Encode(v)
 }
 
+// maxBodyBytes caps an ordinary JSON request body. 1 MiB is far more than any
+// endpoint below needs — it exists so an unauthenticated caller (signup,
+// login, bootstrap) can't make the server buffer an arbitrarily large body
+// before the request is even validated.
+const maxBodyBytes = 1 << 20
+
 func decodeJSON(r *http.Request, v any) error {
+	return decodeJSONLimit(r, v, maxBodyBytes)
+}
+
+// decodeJSONLimit is decodeJSON with an explicit cap, for the one endpoint
+// (the Alertmanager webhook, see maxWebhookBodyBytes) whose real payloads can
+// legitimately be larger than maxBodyBytes.
+func decodeJSONLimit(r *http.Request, v any, limit int64) error {
 	defer r.Body.Close()
+	// w is nil: there is no ResponseWriter here to disable keep-alive with,
+	// which net/http documents as fine — the limit is still enforced, the
+	// connection just isn't closed early on a request that blows past it.
+	r.Body = http.MaxBytesReader(nil, r.Body, limit)
 	return json.NewDecoder(r.Body).Decode(v)
 }