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>

Dockerfile.backend

@@ -0,0 +1,59 @@
+# syntax=docker/dockerfile:1
+
+# ============================================================================
+# Builder stage
+# ============================================================================
+FROM node:22.11.0-slim AS builder
+
+RUN npm install -g corepack@latest && corepack enable
+
+WORKDIR /app
+
+COPY pnpm-workspace.yaml package.json pnpm-lock.yaml ./
+COPY packages/backend/package.json ./packages/backend/package.json
+
+RUN pnpm install --frozen-lockfile --filter @sexy.pivoine.art/backend
+
+COPY packages/backend ./packages/backend
+
+RUN pnpm --filter @sexy.pivoine.art/backend build
+
+RUN pnpm install -rP --filter @sexy.pivoine.art/backend
+
+# ============================================================================
+# Runner stage
+# ============================================================================
+FROM node:22.11.0-slim AS runner
+
+RUN apt-get update && apt-get install -y \
+    dumb-init \
+    ffmpeg \
+    wget \
+    && rm -rf /var/lib/apt/lists/*
+
+RUN userdel -r node && \
+    groupadd -r -g 1000 node && \
+    useradd -r -u 1000 -g node -m -d /home/node -s /bin/bash node
+
+WORKDIR /home/node/app
+
+RUN mkdir -p packages/backend
+
+COPY --from=builder --chown=node:node /app/packages/backend/dist ./packages/backend/dist
+COPY --from=builder --chown=node:node /app/packages/backend/node_modules ./packages/backend/node_modules
+COPY --from=builder --chown=node:node /app/packages/backend/package.json ./packages/backend/package.json
+
+RUN mkdir -p /data/uploads && chown node:node /data/uploads
+
+USER node
+
+ENV NODE_ENV=production \
+    PORT=4000
+
+EXPOSE 4000
+
+HEALTHCHECK --interval=30s --timeout=5s --start-period=20s --retries=3 \
+    CMD wget --no-verbose --tries=1 --spider http://localhost:4000/health
+
+ENTRYPOINT ["dumb-init", "--"]
+CMD ["node", "packages/backend/dist/index.js"]