[Feature] Milestone and Evidence Tracking for the Mentorship Module (GSoC 2026)

### 1. Goal
The goal of this issue is to extend the current **Mentorship Module** by adding a structured way for mentees to track and report their progress against specific project goals. This aligns the OWASP Nest platform with the standard GSoC workflow (Proposal → Midterm → Final Evaluation).

### 2. Core Components
- **Milestone Model:** A new model to define time-bound deliverables (e.g., "Architecture Design," "Core Feature Implementation").
- **Evidence Model:** A mechanism for mentees to submit "evidence" (links to PRs or commits) against a specific milestone.
- **Mentor Review System:** A status-based tracking system (`PENDING`, `APPROVED`, `REJECTED`) for validating student submissions.

### 3. Proposed Technical Changes
**Backend (Django & Strawberry GraphQL):**
- Create `Milestone` and `Evidence` models in `backend/apps/mentorship/models/`.
- Expose these models through the existing Strawberry API by adding `MilestoneNode` and `EvidenceNode`.
- Implement `submitEvidence` and `reviewEvidence` mutations in the GraphQL layer.
- **Admin:** Register both models in the Django Admin for superuser observability.

### 4. Impact
This provides a unified dashboard for tracking mentee progress directly within Nest, making it easier for mentors to evaluate performance and maintain project standards.

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**Note to Maintainers:** I am a GSoC 2026 applicant and have already successfully set up the local environment (including resolving Windows-specific DX blockers). I would like to be assigned to this issue to begin the implementation.