Task #525: Auth endpoint rate limiting (MR-H3)

Closes the remaining High-severity finding from
.local/security/manual-review.md: /api/auth/login,
/api/auth/register, /api/auth/forgot-password,
/api/auth/reset-password and /api/auth/reset-password/verify
accepted unlimited attempts, making remote brute-forcing of
weak passwords feasible against the bcrypt cost-10 store.

Changes
-------
- New artifacts/api-server/src/lib/authRateLimit.ts wraps
  express-rate-limit into route-level middleware:
  * loginIpLimiter         — 10 attempts / 60s per IP
  * loginUsernameLimiter   — 8 attempts / 15min per
                             trim().toLowerCase() username
  * registerIpLimiter      — 5 / hour per IP
  * forgotPasswordIpLimiter— 5 / 15min per IP
  * resetPasswordIpLimiter — 10 / 15min per IP, shared across
                             /reset-password and /reset-password/verify
  * loginLimiters chains IP + username middleware
  All thresholds and windows are env-overridable
  (AUTH_RATE_LIMIT_*_MAX / *_WINDOW_MS).
  Throttled responses are JSON: 429
  { error: "too_many_requests", message: ... }.
  In non-production, requests originating from loopback
  (127.0.0.1, ::1, ::ffff:127.*) skip the limiter so the
  existing test suite — which hammers /auth/login from
  loopback — keeps passing. Production never skips. The
  AUTH_RATE_LIMIT_FORCE=1 escape hatch flips the skip off
  in dev for ad-hoc verification.

- routes/auth.ts: applies the limiters to /auth/register,
  /auth/login, /auth/forgot-password, /auth/reset-password,
  /auth/reset-password/verify. trust proxy was already set
  to 1 in app.ts so X-Forwarded-For from the Replit edge
  drives req.ip in production.

- New artifacts/api-server/tests/auth-rate-limit.test.mjs
  (6 tests). Each test routes through a unique
  X-Forwarded-For 10.x.x.x to bypass the dev loopback skip
  while keeping limiter buckets isolated from one another:
    1. login per-IP: 10 wrong-cred attempts succeed (401),
       11th from the same IP returns 429
    2. login per-username: 8 wrong attempts spread across
       8 fresh IPs all 401, the 9th attempt for the same
       username from yet another fresh IP is 429 — proves
       the username bucket blocks credential stuffing
       even from rotating IPs
    3. forgot-password per-IP: 5 succeed, 6th 429
    4. register per-IP: 5 attempts processed, 6th 429
    5. reset-password (+verify) shared per-IP: 10 mixed
       hits across the two endpoints all processed, 11th 429
    6. loopback regression: 15 login attempts from
       loopback (no X-Forwarded-For) all return non-429,
       proving the dev skip works and existing tests are
       unaffected

Test results
------------
- New file: 6/6 pass.
- Full api-server suite: 327/329 pass. The 2 failures
  (executive-meetings-notifications meeting_created
  socket fan-out, executive-meetings-postpone-race
  postpone-minutes B refetches) are pre-existing
  concurrency flakes — both fail on main before this
  change, both pass when run in isolation, and neither
  touches code modified here.

Architect review
----------------
First round flagged missing register + reset-password
test coverage. Both added in this commit. trust-proxy
hardening flagged as advisory; left as-is because the
existing app.ts already sets trust proxy = 1 to match
the single Replit edge hop, and changing it is out of
this task's scope (separate hardening pass).

Residual risk
-------------
- Per-IP buckets rely on app.set("trust proxy", 1) in
  app.ts. If the deployment topology ever changes to put
  more than one trusted hop in front of the API, the
  trust-proxy value must be raised to match — otherwise
  attackers could spoof X-Forwarded-For to evade per-IP
  limits.
- The username-bucket key is normalized
  (trim().toLowerCase()) but does not collapse Unicode
  homoglyphs. Acceptable for this app: usernames are
  ASCII per RegisterBody validation.

Out of scope
------------
- Helmet, CSRF, session rotation, account-enumeration
  on register, bcrypt cost bump, body-size limits — all
  remain tracked in .local/security/manual-review.md as
  Medium/Low items.

artifacts/api-server/package.json

@@ -24,6 +24,7 @@
     "cors": "^2",
     "drizzle-orm": "catalog:",
     "express": "^5",
+    "express-rate-limit": "^8.5.1",
     "express-session": "^1.19.0",
     "google-auth-library": "^10.6.2",
     "nodemailer": "^8.0.7",