feat: refactor architecture and build course application pipeline wit…

[Emmyt24]

…h email integration

Architecture:

• Separate controllers (pure business logic) from routes (HTTP handlers)
• Slim main.rs to server startup only; extract app builder to app.rs
• Introduce AppState with shared PgPool and EmailService

Application Pipeline:

• Add course application submission (public endpoint)
• Implement admin dashboard with application management
• Support pipeline: pending → interview_invited → waitlisted → accepted/rejected
• Add admin endpoints: accept, reject, enroll, waitlist applications

Email Service:

• Integrate Mailtrap SMTP (transactional stream) via Lettre
• Send interview invitation emails with venue details
• Send enrollment acceptance emails with login credentials
• Send rejection notification emails
• Graceful fallback when email service is unavailable

API & Documentation:

• Add Swagger/OpenAPI docs for all endpoints via utoipa
• Add health check endpoint
• Configure CORS for frontend integration
• Add JWT bearer auth with role-based access (admin, student, mentor)

Infrastructure:

• Add multi-stage Dockerfile for production deployment
• Add .dockerignore
• Configure server to bind 0.0.0.0 with HOST/PORT env vars
• Remove OTP service (replaced by direct auth flow)
• Remove deprecated school/protected route files