fix: align posthog brand seed slug with DB (posthog -> posthog-tam)

The DB already had this brand under slug posthog-tam, causing a domain
unique constraint error when the seed tried to create a new record with
slug posthog for the same domain.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: willwearing

backend/prisma/seeds/brands.ts

@@ -501,7 +501,7 @@ const brands = [
     },
   },
   {
-    slug: "posthog",
+    slug: "posthog-tam",
     name: "PostHog TAM",
     domain: "posthog-tam.vercel.app",
     tagline: "Master PostHog. Technically.",