feat: complete auth and gateway agnostic billing implementation

PRD.md

@@ -74,6 +74,41 @@ The stack is orchestrated using `docker-compose.yml` and consists of 8 distinct
 - **Syllabus Tracker:** Granular chapter-by-chapter completion tracking.
 - **Performance Analytics:** Visual charts tracking accuracy, speed, and subject-wise competence.
 
+### 4.4 Comprehensive API Endpoints
+The backend exposes the following RESTful APIs powered by FastAPI:
+
+**Authentication**
+- `POST /api/auth/register` - Create new student account
+- `POST /api/auth/login` - Authenticate & receive JWT session
+- `POST /api/auth/forgot-password` - Request password reset link
+- `POST /api/auth/otp-request` / `otp-verify` - Phone number authentication
+
+**Users & Profiles**
+- `GET/PUT /api/users/{id}` - Fetch/update user profiles and settings
+- `POST /api/users/{id}/avatar` - Upload profile picture to MinIO
+- `GET /api/users/{id}/attempts` - Retrieve past test history
+
+**CBT Engine**
+- `GET /api/tests` - List active Mock Tests, PYQs, and Weekly Tests
+- `GET /api/tests/{id}/questions` - Fetch test sections and questions
+- `POST /api/attempts/submit` - Auto-grade and record a CBT session
+
+**AI Coach**
+- `GET /api/ai-coach/readiness/{id}` - Predict JEE/NEET rank and college
+- `GET /api/ai-coach/study-plan/today` - Generate daily tasks
+- `GET /api/mistakes/today` - Fetch spaced repetition flashcards (SM-2 algorithm)
+- `POST /api/mistakes/review` - Update mistake mastery based on recall
+- `POST /api/ai-coach/chat/stream` - AI Tutor socket connection
+
+**Subscription & Billing (Gateway Agnostic)**
+- `POST /api/billing/create-order` - Generate payment gateway order IDs
+- `POST /api/billing/verify-payment` - Cryptographically verify webhooks and upgrade tier
+
+**Admin & Parent Systems**
+- `GET/POST/PUT/DELETE /api/admin/*` - Full CRUD for Tests, Users, Coupons, and System Settings
+- `POST /api/parent/register` - Parent account creation
+- `POST /api/parent/link-student` - Link student to parent dashboard
+
 ## 5. Technology Stack Summary
 - **Frontend:** Next.js 14/15, TailwindCSS, Framer Motion, Lucide Icons, Shadcn UI.
 - **Backend:** Python 3.11, FastAPI, Uvicorn.