Designed a 5-table relational PostgreSQL schema with cascading foreign key constraints (users -> clients -> projects -> invoices -> payments) for complete data integrity. Built 15+ REST APIs with Spring Boot covering full CRUD operations, JWT-based stateless authentication, and role-based data isolation ensuring each freelancer accesses only their own data. Implemented Spring Security filter chain with a custom JWT filter to intercept every request, validate tokens, and populate the security context prior to controller execution. Developed a React frontend with protected routing, Axios interceptors for automatic token injection, and a real-time dashboard displaying earnings, active projects, and pending invoices. Built a payment tracking module logging every transaction with mode (UPI/Bank Transfer/Cheque), reference ID, and timestamps – eliminating payment disputes with permanent audit records.