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/service.ts

@@ -0,0 +1,44 @@
+import { randomBytes, randomUUID } from 'node:crypto';
+import type { CoupleRepository } from './repository.js';
+import { CoupleError, type CoupleMutationResponse, type CoupleStateResponse } from './types.js';
+
+const inviteAlphabet = 'ABCDEFGHJKLMNPQRSTUVWXYZ23456789';
+
+export function generateInviteCode(bytes: Uint8Array = randomBytes(8)): string {
+  return Array.from(bytes, (byte) => inviteAlphabet[byte % inviteAlphabet.length]).join('').slice(0, 8);
+}
+
+export class CoupleService {
+  constructor(private readonly repository: CoupleRepository) {}
+
+  getState(userId: string): Promise<CoupleStateResponse> {
+    return this.repository.getState(userId);
+  }
+
+  async create(userId: string): Promise<CoupleMutationResponse> {
+    for (let attempt = 0; attempt < 3; attempt += 1) {
+      const inviteCode = generateInviteCode();
+      try {
+        const couple = await this.repository.create(userId, randomUUID(), inviteCode);
+        return { success: true, couple_id: couple.id, invite_code: couple.invite_code };
+      } catch (error) {
+        if (!(error instanceof CoupleError) || error.code !== 'invite_code_collision') throw error;
+      }
+    }
+    throw new Error('Unable to allocate a unique invite code');
+  }
+
+  async join(userId: string, inviteCode: string): Promise<CoupleMutationResponse> {
+    const canonicalCode = inviteCode.trim().toUpperCase();
+    if (!/^[A-Z0-9]{8}$/.test(canonicalCode)) {
+      throw new CoupleError('invalid_invite_code', 'Invalid invite code format', 400);
+    }
+    const couple = await this.repository.join(userId, canonicalCode);
+    return { success: true, couple_id: couple.id };
+  }
+
+  async cancel(userId: string): Promise<CoupleMutationResponse> {
+    await this.repository.cancel(userId);
+    return { success: true, couple_id: null };
+  }
+}