security: fix CodeQL vulnerabilities (SSRF, XSS, rate limiting)

- Add URL validation to prevent SSRF attacks on notification endpoints
  - Block private IPs (10.x, 172.16-31.x, 192.168.x, 169.254.x)
  - Block localhost and internal hostnames
  - Only allow HTTP/HTTPS protocols
- Add HTML escaping for medication names in email templates (XSS)
- Add stricter rate limiting for auth routes (5 req/15min for login/register)
- Add SSRF protection tests (405 tests total)

backend/package.json

@@ -18,7 +18,7 @@
     "@fastify/helmet": "^13.0.2",
     "@fastify/jwt": "^10.0.0",
     "@fastify/multipart": "^9.3.0",
-    "@fastify/rate-limit": "^10.1.0",
+    "@fastify/rate-limit": "^10.3.0",
     "@fastify/sensible": "^6.0.4",
     "@fastify/static": "^8.3.0",
     "@libsql/client": "^0.10.0",