docs(decisions): add ADR for api endpoint for user exam due dates

docs/decisions/0025-api-endpoint-user-exam-due-dates.rst

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+ADR: API Endpoint for User Exam Due Dates in the Mobile App
+###########################################################
+
+Status
+******
+
+Proposed
+
+Context
+*******
+
+To improve the Open edX mobile experience, we need to provide learners with a fast, accurate list of upcoming exam and assignment due dates across all of their enrolled courses. This is critical for enabling mobile-first learning workflows, proactive planning, and push notifications.
+
+The current approach, which relies on ``get_course_date_blocks()`` and ``modulestore`` traversal, is inefficient, especially for multi-course scenarios. Furthermore, the Open edX community has committed to limiting ``modulestore`` usage in runtime systems (as per ADR 0011) and modularizing content and date management.
+
+Decision
+********
+
+We will reuse and extend the ``edx-when`` application as the unified backend for both course-level and user-level due dates.
+
+Key Implementation Details
+==========================
+
+* **Data Storage**:
+    * Use ``edx-when.ContentDate`` to store canonical due dates published from course metadata.
+    * Use ``edx-when.UserDate`` to store user-specific overrides, including personalized schedules (self-paced) and instructor-granted extensions.
+* **Service Layer**: Implement a dedicated Python service layer within ``edx-when`` to resolve dates. This service will merge ``ContentDate`` and ``UserDate`` records and apply visibility filtering. This ensures the logic is reusable by the LMS, background tasks, and the REST API.
+* **API Endpoint**: Create a new REST API endpoint (proposed at ``/api/edx_when/v1/user-dates/``) that returns a merged, filtered, and sorted list of upcoming due dates for the authenticated user across all enrollments.
+* **Encapsulation**: The REST API will reside within ``edx-when`` to keep it co-located with the models, provided that it does not require circular dependencies on ``edx-platform``.
+
+Scope Limitations
+=================
+
+* **CCX Courses**: Custom Courses for edX (CCX) are excluded from this implementation due to their unique override structure and management complexity.
+
+Consequences
+************
+
+Positive
+========
+
+* **Performance**: Replaces slow runtime block traversal with indexed database queries.
+* **Consistency**: Centralizes date management into a single canonical system for LMS, MFE, and Mobile.
+* **Scalability**: Reduces the load on the ``modulestore`` and aligns with the platform's long-term goal of removing MongoDB dependencies.
+* **Maintainability**: Avoids the creation of redundant data models for "exam dates" specifically.
+
+Negative/Risks
+==============
+
+* **Dependency Management**: We must ensure ``edx-when`` does not import code from ``edx-platform`` to maintain clean encapsulation. If complex platform logic is required, a thin wrapper API may need to be hosted in ``edx-platform`` instead.
+
+Rejected Alternatives
+*********************
+
+1. **Continue Using XBlock Metadata**
+   * *Description*: Store all start, due, and grace period fields in ``modulestore``.
+   * *Reason for Rejection*: No support for user-level overrides without course republishing; cannot support dynamic scheduling or extensions effectively.
+
+2. **Use Only the 'Schedules' App**
+   * *Description*: Centralize all date logic through ``schedules.Schedule``.
+   * *Reason for Rejection*: Not designed for block-level (exam-specific) due dates; would require significant refactoring of upstream APIs.
+
+3. **Compute Dates On-the-Fly at Runtime**
+   * *Description*: Dynamically calculate due dates based on enrollment and pacing without persistence.
+   * *Reason for Rejection*: Degrades performance at scale and provides no audit trail for extension history.
+
+4. **Separate Microservice**
+   * *Description*: Introduce a new service for exam dates.
+   * *Reason for Rejection*: Adds unnecessary operational overhead and duplicates functionality already available in ``edx-when``.