feat(posts): add backend endorsement system (tests failing)

[michelleguo04]

This branch cannot be merged into main because automated tests are failing. Additional debugging or test updates are required before merge approval.

Justification: I have tried several approaches in implementing the endorsement function. They all could not get over a test. My local npm run test passes on every try and I have tried rerunning the test on github, yet it kept giving the same test error related to post/emails on the github tests. Teammate Jonathan has fixed the post/emails bug in his latest version on main, but it reappears in my section. The committed code in this section might seem a lot, but they are smaller sections that were implemented previously on separate branches.

Change description:

Implements the complete backend functionality for post endorsements, including API routes, data handling, and integration across posts, topics, and search modules.

Issue #4
Issue #5
Isusue #7

BACKEND

src/posts/endorsements.js (New File)
Created the core endorsement logic module
Implemented:
Posts.endorse() — allows moderators/admins to endorse posts
Posts.unendorse() — removes endorsements
Posts.hasEndorsed() — checks if a user has endorsed post(s)
Posts.getEndorsers() — retrieves all users who endorsed a post
Added permission checks for administrators and moderators
Uses database sets (pid:${pid}:endorsers) to track endorsers
Fires plugin hooks:
action:post.endorse
action:post.unendorse
src/database/migrations/2025-01-27-add-endorsements-to-posts.js (New File)
Added endorsements field to post objects
Ensures all existing posts have the field initialized

API

src/api/posts.js
Added API methods for endorse and unendorse operations
Integrated these with the core endorsements module
src/api/helpers.js
Updated helper functions to support endorsement-related API responses
src/controllers/write/posts.js
Added:
Posts.endorse() controller handler
Posts.unendorse() controller handler
Both use the mock() helper for request preparation
src/routes/write/posts.js
Registered new routes:
PUT /api/v3/posts/:pid/endorse
DELETE /api/v3/posts/:pid/endorse
Routes require authentication and post assertion middleware

Database

src/posts/data.js
Updated post data retrieval to include endorsement info
src/posts/summary.js
Added endorsements to post summary retrieval
Added stripEndorsements option for search API compatibility
Handles endorsement visibility in anonymous post scenarios
src/posts/index.js
Integrated the endorsements module into the main Posts module
src/posts/diffs.js
Updated to handle endorsement data during post edits/diffs

[thejonathangu]

Its not passing the flaky test with validation email