feat: Spam Reporting Feature

[zp6]

Closes #1148

Complete spam reporting system ($100 bounty):

• Report model (reporter, target_type, target_id, reason, status)
• API: POST /api/v2/reports (create), GET /api/v2/reports (list), GET /api/v2/reports/:id (show)
• Frontend: report button directive + modal template
• Angular resource for Report API
• DB migration + controller specs + model specs
• 412 lines added

GitHub: @zp6

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}