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