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/backend/package.json

@@ -0,0 +1,51 @@
+{
+  "name": "@sexy.pivoine.art/backend",
+  "version": "1.0.0",
+  "type": "module",
+  "private": true,
+  "scripts": {
+    "dev": "tsx watch src/index.ts",
+    "build": "tsc",
+    "start": "node dist/index.js",
+    "db:generate": "drizzle-kit generate",
+    "db:migrate": "drizzle-kit migrate",
+    "db:studio": "drizzle-kit studio",
+    "migrate": "tsx src/scripts/data-migration.ts"
+  },
+  "dependencies": {
+    "@fastify/cookie": "^11.0.2",
+    "@fastify/cors": "^10.0.2",
+    "@fastify/multipart": "^9.0.3",
+    "@fastify/static": "^8.1.1",
+    "@pothos/core": "^4.4.0",
+    "@pothos/plugin-errors": "^4.2.0",
+    "argon2": "^0.43.0",
+    "drizzle-orm": "^0.44.1",
+    "fastify": "^5.4.0",
+    "fluent-ffmpeg": "^2.1.3",
+    "graphql": "^16.11.0",
+    "graphql-scalars": "^1.24.2",
+    "graphql-ws": "^6.0.4",
+    "graphql-yoga": "^5.13.4",
+    "ioredis": "^5.6.1",
+    "nanoid": "^5.1.5",
+    "nodemailer": "^7.0.3",
+    "pg": "^8.16.0",
+    "slugify": "^1.6.6",
+    "uuid": "^11.1.0"
+  },
+  "pnpm": {
+    "onlyBuiltDependencies": [
+      "argon2"
+    ]
+  },
+  "devDependencies": {
+    "@types/fluent-ffmpeg": "^2.1.27",
+    "@types/nodemailer": "^6.4.17",
+    "@types/pg": "^8.15.4",
+    "@types/uuid": "^10.0.0",
+    "drizzle-kit": "^0.31.1",
+    "tsx": "^4.19.4",
+    "typescript": "^5.9.3"
+  }
+}