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/mobile/src/lib/coupleApi.ts

@@ -0,0 +1,23 @@
+import type { Couple, Profile } from "@/types";
+import { apiRequest } from "./apiClient";
+
+export interface CoupleStateResponse {
+    couple: Couple | null;
+    partner: Profile | null;
+}
+
+export interface CoupleMutationResponse {
+    success: true;
+    couple_id: string | null;
+    invite_code?: string;
+}
+
+export const coupleApi = {
+    getState: () => apiRequest<CoupleStateResponse>("/v1/couple"),
+    create: () => apiRequest<CoupleMutationResponse>("/v1/couple", { method: "POST", body: {} }),
+    join: (inviteCode: string) => apiRequest<CoupleMutationResponse>("/v1/couple", {
+        method: "POST",
+        body: { invite_code: inviteCode },
+    }),
+    cancel: () => apiRequest<CoupleMutationResponse>("/v1/couple", { method: "DELETE" }),
+};