feat(settings): expose the API rate limiter in the Security tab (#1338)

* feat(settings): expose the API rate limiter in the Security tab

The general per-IP limiter had six settings in app_settings and a
backend route to write them, and no screen. Installs ran on the code
fallback — 300 requests per 15 minutes per IP — with no way to see it,
which is how issue 1287 played out: a 546-photo gallery exhausted the
budget for one viewer and the operator learned about the setting from
a grep of the backend log.

Security tab: a card with the six settings, the validation ranges the
route enforces, a one-line explanation per field, and a note that the
unit is the client IP — an office or household behind one NAT shares
a budget, and behind a proxy TRUST_PROXY has to cover the proxy or
every visitor shares its address. The tab's Save button saves the
limiter through its own route. The limiter values are checked against
the route's ranges before anything is written and the limiter is
written first, so a rejected value cannot leave the password/session
settings half-saved behind a failure toast.

Backend, three things the screen needed:

- The settings read fills the six keys with the code defaults when
  they have no row, so the form shows the budget in force rather than
  an empty field; the defaults live in one exported constant the
  limiter itself reads.
- The write route upserts instead of updating: on a fresh install,
  which has no rows, the old UPDATE matched nothing and the route
  answered 200 while changing nothing.
- The live limiter instances move into rateLimitService and the
  write route rebuilds them. express-rate-limit fixes windowMs when an
  instance is built — max and skip re-read the settings per request,
  the window does not — so a saved window used to apply only after a
  restart. The gates in server.js resolve the instance per request
  through the service's getters. A rebuild starts fresh counters,
  which on a settings change is acceptable. The limiters get explicit
  MemoryStores and a rebuild shuts the superseded ones down, because a
  store keeps a cleanup interval alive for as long as it exists and
  dropping the reference alone would leak one timer per save.

Tests: the read surfaces defaults and honours the key filter; the
write creates rows on a fresh database, the limiter sees the values
immediately and hands the gates a fresh instance; existing rows are
updated not duplicated; out-of-range values are rejected. The tab
renders the values, edits through the hook state, carries the ranges,
saves with the tab's button, and the pre-write validation accepts the
bounds and rejects outside them and cleared fields.

Relates to issue 1337

* docs(security): point the rate limiter doc at the Security tab and the upsert

---------

Co-authored-by: Paul Nothaft <[email protected]>

backend/docs/SECURITY_LOGGING.md

@@ -87,7 +87,7 @@ When rate limits are exceeded, the following is logged:
 
 ## Configuration Settings
 
-The general limiter reads these keys from `app_settings` (cached for 60 seconds). There is no admin screen for them yet; they are set through `PUT /api/admin/settings/security/rate-limit` (all six fields required) or directly in the table, JSON-encoded. The defaults below are what applies when a key has no row — a fresh install has none. Note that the route only updates rows that already exist: on a fresh install it answers 200 without writing anything, so insert the six rows directly first.
+The general limiter reads these keys from `app_settings` (cached for 60 seconds). They are edited in the admin panel under Settings → Security (the API rate limiter card), which saves through `PUT /api/admin/settings/security/rate-limit` (all six fields required). The route upserts, so a fresh install needs no rows first, and the live limiters are rebuilt on save, so a new window applies without a restart. The defaults below are what applies when a key has no row; the settings read fills them in, so the form shows the budget in force.
 
 | Setting | Default | Range | Description |
 |---------|---------|-------|-------------|