Add abuse report button to video detail page

[kelleyglenn]

Summary

Logged-in users should be able to report inappropriate or incorrect video submissions from the video detail page.

Acceptance Criteria

• "Report" button/link visible to logged-in users on the video detail page
• Opens a modal with reason selection (Spam, Inappropriate, Copyright, Misinformation, Other)
• Optional text description field
• Submits via POST /api/v1/moderation/reports
• Shows confirmation toast on success
• Button hidden for anonymous users

Related Issues

• AccountabilityAtlas#17 — Revisit abuse report workflow automation (backend side)
• Moderation-service already implements POST /moderation/reports endpoint