fix(api): add tiered rate limiting to prevent polling lockouts (#460)

* fix(api): add tiered rate limiting to prevent polling lockouts

Replace the single global rate limiter (100 req/min/IP) with a tiered
system that separates high-frequency polling endpoints from standard
API traffic:

- Polling tier (300/min): /stats, /system/stats, /stacks/statuses,
  /metrics/historical, /health, /meta, /auth/status, /auth/sso/providers,
  /license. Exempt from the global limiter but governed by their own
  safety net to prevent resource exhaustion.
- Standard tier (200/min): All other endpoints, raised from 100.
- Webhook tier (500/min): POST /webhooks/:id/trigger, dedicated limiter
  for CI/CD platforms sharing datacenter IPs.
- Auth tier: Unchanged (5-10 attempts / 15 min).

Enterprise adaptations:
- Authenticated requests keyed by user session (JWT sub/username) instead
  of IP, preventing shared NAT/VPN environments from pooling budgets.
- Internal node-to-node traffic (node_proxy tokens) bypasses all rate
  limiters entirely.

Includes comprehensive stress tests (21 cases) validating tier
separation, node proxy bypass, and per-user keying.

* chore(deps): bump axios to 1.15.0 to fix SSRF vulnerability

Addresses GHSA-3p68-rc4w-qgx5 (NO_PROXY hostname normalization bypass).

docs/reference/security-advisories.mdx

@@ -13,7 +13,7 @@ A comprehensive security audit was performed before the Sencho paid tiers launch
 
 - **Path traversal prevention** in `env_file` resolution, preventing reads outside the compose directory
 - **Stack name validation** enforced on all routes, rejecting names with special characters or path components
-- **Global API rate limiting** to protect against brute-force and abuse
+- **Tiered API rate limiting** with per-user session keying to protect against brute-force and abuse
 
 ### Authentication and secrets