Merge improvements/fork-harvest into main

Fork-harvest brings: the ip-api.com geo fix, credential bundle scan,
mock gateways out of production, JIT compiler dropped (1.55->1.04 MB),
host hardening, provider-agnostic identity + VK/Yandex + account linking,
and the backend contracts consolidated into one BACKEND-INTEGRATION.md.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

# Conflicts:
#	docs/backend/BACKEND-HANDOFF.md
#	docs/backend/TRACK-S-SECURITY-RBAC-CONTRACT.md

docs/DEPLOYMENT.md

@@ -5,7 +5,7 @@ separate developer's responsibility. API hostnames are separate reverse proxies
 and return `502` until their configured upstream exists (production currently
 defaults to `https://127.0.0.1:445`).
 
-**Multi-tenant, one bundle.** Every customer domain is served by the same build. The SPA resolves its tenant from the `Host` header ([BACKEND-HANDOFF §1a](backend/BACKEND-HANDOFF.md)). One deploy updates every domain simultaneously — there is no per-tenant build and no per-tenant deploy.
+**Multi-tenant, one bundle.** Every customer domain is served by the same build. The SPA resolves its tenant from the `Host` header ([BACKEND-HANDOFF §1a](backend/BACKEND-INTEGRATION.md)). One deploy updates every domain simultaneously — there is no per-tenant build and no per-tenant deploy.
 
 The SPA derives one API origin from the storefront's base domain:
 `example.com`, `store1.example.com`, and `www.example.com` all use
@@ -79,6 +79,28 @@ password, writes the catch-all nginx config, opens 80/443/OpenSSH in ufw, and
 installs a root-owned, argument-validating API-domain helper. The deploy user may
 run that helper and reload nginx, but cannot replace the helper.
 
+It also applies host hardening (added 2026-08-21, FH-D.3) — three drop-in files,
+so a re-run replaces its own config and never edits a distro file in place:
+
+| File | Effect |
+|---|---|
+| `/etc/ssh/sshd_config.d/10-marketplaces-hardening.conf` | Password and keyboard-interactive auth off, root key-only, no agent/X11 forwarding, `MaxAuthTries 3`, 30 s login grace |
+| `/etc/fail2ban/jail.d/marketplaces.local` | `sshd`, `nginx-http-auth`, `nginx-bad-request` jails — 5 failures in 10 min, 1 h ban |
+| `/etc/sysctl.d/99-marketplaces-hardening.conf` | No redirects or source routing, reverse-path filtering, SYN cookies, forwarding off, restricted kernel pointers and dmesg |
+
+Both accounts on this host are key-only by construction, so disabling password
+auth cannot lock anyone out — it only closes unlimited guessing against a
+credential nobody intended to exist. The script runs `sshd -t` before reloading
+and removes its own drop-in if the test fails, because a bad sshd config taking
+effect on a remote box is how people lock themselves out permanently.
+
+Confirm after provisioning:
+
+```bash
+sudo fail2ban-client status sshd
+sudo sshd -T | grep -E 'passwordauthentication|permitrootlogin|maxauthtries'
+```
+
 Verify before continuing:
 
 ```bash
@@ -149,7 +171,7 @@ sudo bash setup-wildcard-tls.sh \
 
 Wildcards require DNS-01 validation, so certbot must write a `_acme-challenge` TXT record. With a provider plugin (`cloudflare`, `route53`) renewal is unattended. `--dns manual` works but prompts for a TXT record at **every** renewal — fine to prove the setup out, not acceptable as a steady state.
 
-**Hostinger has no certbot plugin.** If DNS lives there: either move DNS to a provider that has one (Cloudflare is free, minutes of work), or drive issuance from the [Phase 9](backend/PHASE-9-TENANT-REGISTRY-DOMAINS-CONTRACT.md) domain-automation API once it exists.
+**Hostinger has no certbot plugin.** If DNS lives there: either move DNS to a provider that has one (Cloudflare is free, minutes of work), or drive issuance from the [Phase 9](backend/BACKEND-INTEGRATION.md) domain-automation API once it exists.
 
 ### 4.2 Reconciler — tenants on their own domains