Need a backend for a simple API (Google Cloud, Firebase Dynamic Links)

Backend REST API for a mobile application with a subscription system and referral program without registration for a mobile application on Flutter.

Milestone 1: Data Preview API (Day 1)

Goal: there is a third-party API that solves 99% of tasks. There is a lot of unnecessary data, we need to keep only the requests we need and ensure that the requests go through our service.

Functionality

• Endpoint for obtaining preview data on request
• Validation and availability check of data through the external API (the API is already available, I will provide access)
• Caching results (TTL 1 hour)
• Error handling (404, 403)

Technical Requirements

• POST endpoint with validation of input parameters
• Integration with external REST API (details in private message)
• PostgreSQL for caching
• Return structured JSON response

Milestone 2: Subscription Tracking + Push Notifications (Day 2)

Goal: the API does not allow comparing past results or seeing changes from the previous result. There are also no timestamps. This needs to be added to our service so that it is possible to see if anything has changed between the previous and new request. I will provide more information in private message.

Functionality

• Activation of tracking after subscription purchase
• Daily cron job for automatic change checking (00:00 UTC)
• Comparison of snapshots and identification of new records
• Sending personalized push notifications via Firebase FCM
• Automatic deactivation upon subscription expiration

Technical Requirements

• Endpoints: activate, get results, deactivate
• PostgreSQL tables for storing snapshots
• Cloud Scheduler for cron job
• Firebase Cloud Messaging integration
• Data comparison logic and marking NEW records

Milestone 3: Referral System (Day 3)

Goal: to add a simple referral program to the mobile application without registration. There should be a unique deep link for each user that opens our application in the App Store or Google Play.

Functionality

• Generation of unique referral codes
• Firebase Dynamic Links for deep linking
• Automatic redirect to App Store/Google Play
• Tracking activations without registration (device_id based)
• Unlocking bonuses after 3+ activations

Technical Requirements

• Endpoints: generate, activate, status, claim bonus
• Firebase Dynamic Links setup
• PostgreSQL for storing referrals
• Fraud protection (1 device = 1 activation)
• Rate limiting

Deliverables

✅ Backend deployed on Google Cloud Run
✅ PostgreSQL database configured
✅ Cron job is working
✅ Firebase FCM + Dynamic Links integrated
✅ Postman collection
✅ API documentation
✅ .env.example and deployment guide

I will send a more detailed technical specification in private message. Another developer will be accessing this service (I am currently looking), so it may be necessary to sync or refine something in the future.

Please provide an approximate cost and timeline that you estimate for this task. I need someone who understands what needs to be done (especially with the referral system), as I will not be able to help/explain technically.

Desired timeline: within 2-4 days.