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/.env.example

@@ -0,0 +1,8 @@
+# Local development only. Docker Compose already supplies these defaults.
+# Do not reuse these credentials for a deployed environment.
+# Database name/user/password are deliberately fixed by compose.yaml so shell
+# variables from another environment cannot accidentally change this stack.
+SAUCI_POSTGRES_PORT=54320
+
+# The API runs on the host during development, outside Docker Compose.
+SAUCI_API_PORT=3003