feat: replace Directus with custom Node.js GraphQL backend

Removes Directus 11 and replaces it with a lean, purpose-built backend:
- packages/backend/: Fastify v5 + GraphQL Yoga v5 + Pothos (code-first)
  with Drizzle ORM, Redis sessions (session_token cookie), argon2 auth,
  Nodemailer, fluent-ffmpeg, and full gamification system ported from bundle
- Frontend: @directus/sdk replaced by graphql-request v7; services.ts fully
  rewritten with identical signatures; directus.ts now re-exports from api.ts
- Cookie renamed directus_session_token → session_token
- Dev proxy target updated 8055 → 4000
- compose.yml: Directus service removed, backend service added (port 4000)
- Dockerfile.backend: new multi-stage image with ffmpeg
- Dockerfile: bundle build step and ffmpeg removed from frontend image
- data-migration.ts: one-time script to migrate all Directus/sexy_ tables

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/frontend/package.json

@@ -40,8 +40,9 @@
     "vite-plugin-wasm": "3.5.0"
   },
   "dependencies": {
-    "@directus/sdk": "^21.1.0",
     "@sexy.pivoine.art/buttplug": "workspace:*",
+    "graphql": "^16.11.0",
+    "graphql-request": "^7.1.2",
     "javascript-time-ago": "^2.6.4",
     "media-chrome": "^4.18.0",
     "svelte-i18n": "^4.0.1"