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.

infra/backend/compose.yaml

@@ -0,0 +1,23 @@
+name: sauci-local
+
+services:
+  postgres:
+    image: postgres:17-alpine
+    environment:
+      POSTGRES_DB: sauci
+      POSTGRES_USER: sauci
+      POSTGRES_PASSWORD: sauci_local_only
+    ports:
+      - "127.0.0.1:${SAUCI_POSTGRES_PORT:-54320}:5432"
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U $${POSTGRES_USER} -d $${POSTGRES_DB}"]
+      interval: 2s
+      timeout: 3s
+      retries: 20
+      start_period: 5s
+    volumes:
+      - sauci-postgres-data:/var/lib/postgresql/data
+    restart: unless-stopped
+
+volumes:
+  sauci-postgres-data: