Taariq: Add Lesson Plan Builder UI and update routes

[TaariqMansurie]

Description

This PR implements the backend API and database schemas for the "Assign Lesson Plan" feature. It introduces the endpoints required to assign lesson plan tasks to eligible students and logs the assignment history.
Implements # (PRIORITY HIGH) Phase 4 - Assign Lesson Plan - Backend API (WIP Taariq)

Related PRS (if any):

This backend PR is related to the frontend PR for the UI integration.
To test this backend PR, you need to checkout the corresponding frontend PR.
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Main changes explained:

• Create models/educationTask.js for the schema defining assigned student tasks.
• Create models/lessonPlanLog.js for the schema tracking assignment and edit history.
• Create routes/educatorRouter.js for the /api/educator/assign-tasks endpoint (handles student eligibility and bulk task creation) and the /logs/:lessonPlanId endpoint.
• Update routes.js to register the new /api/educator routes.
  …

How to test:

1. check into current branch
2. do yarn install or npm install and start the local server
3. log in as an admin or educator user on the frontend (using the related frontend branch)
4. go to the Lesson Plan Builder page and trigger an assignment (manual or auto)
5. verify the API returns a 200 success response with the correct assigned/skipped counts
6. verify in the database (MongoDB) that new documents are created in the educationtasks and lessonplanlogs collections.

Screenshots or videos of changes:

Note:

Include the information the reviewers need to know.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud