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>

package.json

@@ -5,11 +5,11 @@
 	"main": "index.js",
 	"scripts": {
 		"test": "echo \"Error: no test specified\" && exit 1",
-		"build:bundle": "git pull && pnpm install && pnpm --filter @sexy.pivoine.art/bundle build",
 		"build:frontend": "git pull && pnpm install && pnpm --filter @sexy.pivoine.art/frontend build",
-		"dev": "pnpm build:bundle && docker compose up -d && pnpm --filter @sexy.pivoine.art/frontend dev",
-		"schema:export": "docker compose exec directus node /directus/cli.js schema snapshot --yes /tmp/snapshot.yml && docker compose cp directus:/tmp/snapshot.yml ./directus.yml && docker compose exec db pg_dump -U sexy --schema-only sexy > schema.sql",
-		"schema:import": "docker compose exec -T postgres psql -U sexy sexy < schema.sql && docker compose cp ./directus.yml directus:/tmp/snapshot.yml && docker compose exec directus node /directus/cli.js schema apply --yes /tmp/snapshot.yml"
+		"build:backend": "git pull && pnpm install && pnpm --filter @sexy.pivoine.art/backend build",
+		"dev:data": "docker compose up -d postgres redis",
+		"dev:backend": "pnpm --filter @sexy.pivoine.art/backend dev",
+		"dev": "pnpm dev:data && pnpm dev:backend & pnpm --filter @sexy.pivoine.art/frontend dev"
 	},
 	"keywords": [],
 	"author": {