feat(backend): add standalone data plane

Replace the Supabase application data plane with the Node/PostgreSQL API, migrate mobile, admin, web, and MCP callers, add controlled cutover tooling, and define hardened Dokploy infrastructure.

apps/api/Dockerfile

@@ -0,0 +1,28 @@
+FROM node:20-bookworm-slim AS build
+WORKDIR /workspace
+COPY package.json package-lock.json ./
+COPY apps/api/package.json ./apps/api/package.json
+COPY packages/shared/package.json ./packages/shared/package.json
+RUN --mount=type=cache,target=/root/.npm \
+  npm ci --workspace @sauci/api --include-workspace-root=false --ignore-scripts
+COPY apps/api ./apps/api
+COPY packages/shared ./packages/shared
+RUN npm run build --workspace @sauci/api
+RUN npm prune --omit=dev --workspace @sauci/api --include-workspace-root=false --ignore-scripts
+
+FROM node:20-bookworm-slim AS runtime
+ENV NODE_ENV=production PORT=3003
+WORKDIR /app
+RUN mkdir -p /data/media && chown node:node /data/media
+COPY --from=build /workspace/node_modules ./node_modules
+# npm may place 0.x dependency upgrades in the workspace-local tree. Overlay
+# that pruned tree so runtime resolution matches the workspace lockfile.
+COPY --from=build /workspace/apps/api/node_modules ./node_modules
+COPY --from=build --chown=node:node /workspace/apps/api/package.json ./package.json
+COPY --from=build --chown=node:node /workspace/apps/api/dist ./dist
+COPY --from=build --chown=node:node /workspace/apps/api/drizzle ./drizzle
+USER node
+EXPOSE 3003
+HEALTHCHECK --interval=15s --timeout=3s --start-period=10s --retries=3 \
+  CMD ["node", "-e", "fetch('http://127.0.0.1:3003/health/live').then(r=>{if(!r.ok)process.exit(1)}).catch(()=>process.exit(1))"]
+CMD ["node", "dist/index.js"]