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/src/domains/couples/routes.ts

@@ -0,0 +1,53 @@
+import type { ApiErrorResponse } from '@sauci/shared';
+import type { Hono } from 'hono';
+import { z } from 'zod';
+import type { AuthIdentity } from '../../auth.js';
+import { CoupleService } from './service.js';
+import { CoupleError } from './types.js';
+
+type Variables = { identity: AuthIdentity };
+type AuthenticatedApp = Hono<{ Variables: Variables }>;
+
+const joinBody = z.object({ invite_code: z.string() }).strict();
+
+function error(code: string, message: string): ApiErrorResponse {
+  return { error: { code, message } };
+}
+
+async function route<T>(operation: () => Promise<T>): Promise<
+  { ok: true; value: T } | { ok: false; body: ApiErrorResponse; status: 400 | 404 | 409 }
+> {
+  try {
+    return { ok: true, value: await operation() };
+  } catch (cause) {
+    if (cause instanceof CoupleError) {
+      return { ok: false, body: error(cause.code, cause.message), status: cause.status };
+    }
+    throw cause;
+  }
+}
+
+export function registerCoupleRoutes(app: AuthenticatedApp, service: CoupleService): void {
+  app.get('/v1/couple', async (c) => {
+    const result = await route(() => service.getState(c.get('identity').id));
+    return result.ok ? c.json(result.value) : c.json(result.body, result.status);
+  });
+
+  app.post('/v1/couple', async (c) => {
+    const rawBody: unknown = await c.req.json().catch(() => ({}));
+    if (typeof rawBody === 'object' && rawBody !== null && Object.keys(rawBody).length === 0) {
+      const result = await route(() => service.create(c.get('identity').id));
+      return result.ok ? c.json(result.value, 201) : c.json(result.body, result.status);
+    }
+
+    const parsed = joinBody.safeParse(rawBody);
+    if (!parsed.success) return c.json(error('invalid_invite_code', 'A valid invite_code is required'), 400);
+    const result = await route(() => service.join(c.get('identity').id, parsed.data.invite_code));
+    return result.ok ? c.json(result.value) : c.json(result.body, result.status);
+  });
+
+  app.delete('/v1/couple', async (c) => {
+    const result = await route(() => service.cancel(c.get('identity').id));
+    return result.ok ? c.json(result.value) : c.json(result.body, result.status);
+  });
+}