Tier 1 hardening: trustProxy + per-IP rate limit + anonymity flag

- Fastify public app now passes `trustProxy: true` so request.ip resolves to
  the real client IP from X-Forwarded-For when behind Cloud Run / a load
  balancer. Without this the per-IP rate limit was either useless or
  collapsed all users into one shared LB IP.
- The `anonymity_enabled` config row + JS default + migration seed now
  default to `false`. The flag is dead code (no business logic ever
  consumed it) and the actual rule is simpler than the toggle implied:
  mitras always see the customer's chosen call_name; only phone+email
  are private. The whole feature is queued for rip-out as a separate
  cleanup pass.

The per-IP OTP rate limit (10/hr) was also effectively disabled by
upserting `app_config.otp_max_per_ip_per_hour = 1000000` — a runtime
config change, not a code change. Per-phone (3/hr) + Fazpass cost
remains the real abuse gate.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/db/migrate.js

@@ -60,7 +60,7 @@ const migrate = async () => {
 
   await sql`
     INSERT INTO app_config (key, value)
-    VALUES ('anonymity', '{"enabled": true}')
+    VALUES ('anonymity', '{"enabled": false}')
     ON CONFLICT (key) DO NOTHING
   `