deps: update posthog requirement from >=7.0.0 to >=7.18.0 in /backend

Updates the requirements on [posthog](https://github.com/posthog/posthog-python) to permit the latest version.
- [Release notes](https://github.com/posthog/posthog-python/releases)
- [Changelog](https://github.com/PostHog/posthog-python/blob/main/CHANGELOG.md)
- [Commits](PostHog/posthog-python@v7.0.0...7.18.0)

---
updated-dependencies:
- dependency-name: posthog
  dependency-version: 7.15.4
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

backend/pyproject.toml

@@ -31,7 +31,7 @@ dependencies = [
     "stripe>=15.1.0",
     "resend>=2.30.0",
     "httpx>=0.28.0",
-    "posthog>=7.0.0",
+    "posthog>=7.18.0",
 ]
 
 [dependency-groups]