auth: sign-in limits per address and name; successes free (audit A1, A8, A9)

Security audit 2026-10-17, findings A1 (medium), A8, A9 (low), stage 3.
Under the shipped Apache-on-host deployment with TRUSTED_PROXY_CIDR empty,
every visitor has the Docker gateway's address, and every public limit was
keyed on the address alone: ten POSTs to /login per 15 minutes (any body,
successes counted) locked out every administrator; likewise /login/reset,
the link pages and /setup/. IPv6 was keyed per /128.

Now (internal/web/auth/limitkeys.go):
- sign-in counts per address and username (10 / 15 min) and per address
  alone at 100; a right password refunds both (rateLimiter.Refund);
- a reset request counts per address and username (5) and per address
  alone (50); a link's page per link, by token hash (30), and per address
  alone (300); setup per address as before;
- an IPv6 address is keyed by its /64 (limitAddr);
- proxyWatch: when the first 20 requests after a start all come from one
  private or loopback peer with X-Forwarded-For and no proxy is trusted,
  the log says so once with the TRUSTED_PROXY_CIDR value to set;
- a refused sign-in or a lockout is logged, five lines per address in 15
  minutes, the typed name printable and cut (refusalLog, logName).
The shipped compose cannot know the gateway, so the guide's install step
prints it (docker network inspect) and says to set it.

Tests: TestLoginBehindOneUntrustedProxyKeepsOthersIn (the reader's case),
TestLoginSuccessesDoNotCount, TestIPv6IsLimitedPerSlash64,
TestProxyWatchAndRefusalsAreLogged; the existing lockout tests pass.

Docs: security.md § First-run setup, § Authentication; guide § Full
deployment; CHANGELOG; plan. Also the I1 plan line's Done note.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/guide.md

@@ -183,6 +183,19 @@ docker compose up -d
 The nginx/Caddy/Traefik fragments already include `docker compose up -d` —
 skip this if you ran one of those.
 
+With Apache on the host, the panel sees every visitor at the Docker
+network's gateway. Tell it that address is your proxy, so that each visitor
+has a sign-in limit of their own, and recreate the container:
+
+```sh
+docker network inspect "$(basename "$PWD")_default" -f '{{(index .IPAM.Config 0).Gateway}}'
+```
+
+Put what it prints into `.env` as `TRUSTED_PROXY_CIDR=` and run `docker
+compose up -d` again. Until then the panel's log says, after the first
+requests, that every visitor shares one limit and what to set; Server ›
+Health shows the same under *Your address*.
+
 **Get the setup URL** — open it in a browser to create the admin account (see
 [Initial setup](#initial-setup)):