Senior Flutter Developer - Codebase Takeover & API Calibration

e are seeking an elite, highly experienced Senior Flutter/Dart Developer or Agency to take over a clean, well-structured mobile codebase. The core architecture is completely established on Firebase (Firestore/Authentication/Storage) and GitHub. Important: This is NOT a build-from-scratch project. The vast majority of the core engine work is already 100% complete and sitting inside our pristine master GitHub branch from 2 days ago. We do not need front-end design overhauls. We need a precision backend systems engineer to calibrate existing API streams, correct state-management strings, and execute a final polish checklist for a strict $300 fixed budget. Core Responsibilities & Milestone Scope of Work: Phase 1: API Calibration, Pricing Calculations, & Refinery Audit * Pricing Engine & 9-Cent Rendering Calibration: Audit and verify the automated cost-calculation logic across the storefront and generation engines. You will calibrate the pricing pipelines to ensure the platform accurately processes and visually renders our 9-cent transaction tiers flawlessly on mobile and web screen glass, removing any infinite price-calculation loops. * AI Beat Maker & Refinery Stream (Audit & Data Link): The core generative audio engines are built, but the "pick a beat" selection loop inside the refinery module may be disconnected or uncommitted on the master repository branch. You will audit this data flow, link the beat selection state-management parameters cleanly to our Firestore tables, and ensure users can seamlessly select tracks without encountering connection drops. * Audio Terminology & Fader Correction: The previous contractor flipped industry-standard audio mapping strings backwards. They incorrectly labeled the overarching master panning layout as "Track Controls" and named the independent stem tracks (Drums, Bass, Melody, Harmony, FX) as "Volume Faders." You will cleanly standardize these state-management variables so that the instrument volume faders stream and isolate flawlessly. * Mastering Pipeline Stabilization: The mastering section was previously operational but is currently failing on mobile builds due to this specific stem state-management mismatch. You will recalibrate the asynchronous processing queries so that mastered tracks render correctly on mobile screen glass. * RevenueCat Payment Walls: Finalize the integration and testing of our multi-tier subscription and credit-based payment systems across iOS, Android, and Web clients. Phase 2: "Tighten Loose Ends" (Pre-Launch Final Checklist) *This phase is strictly minor polish to clean up existing UI layouts before deployment. It is expected to take minimal time:* * UI/UX Layout Details: Standardizing static layout text blocks, cleaning up neon banner visual boundaries, and ensuring the existing Arena leaderboard displays smoothly via its top-right drop-down header placement. * Profile Account Badges: Mapping a basic frontend layout display rule to show three custom static image badges on user profile cards when an account role tag is active. * Admin Tools Verification: Doing a quick audit of the backend admin checks to confirm our existing profile delete scripts function correctly for master admin profiles. Technical Requirements: * 5+ years of verified, advanced experience with Flutter & Dart. * Deep expertise in Firebase Suite (Firestore architecture, Cloud Functions, Security Rules). * Flawless production experience integrating native mobile payment infrastructures (RevenueCat, App Store Connect, Google Play Console). * Sovereignty: 100% of code commits and architecture passes must occur directly inside our private GitHub organization. Local-only deployments or uncommitted external web hosting pushes will not be accepted.