Add moderation and reporting workflow for comments and solutions

[Mkalbani]

Description

With discussion/comments and solution submission both live, there's no way for users to flag abusive or low-quality content, and no moderator workflow to act on reports. Add a reporting system with a moderation queue.

Acceptance Criteria

• POST /comments/:id/report and POST /solutions/:id/report endpoints accept a reason code
• Reports are deduplicated so the same user reporting the same content twice doesn't create duplicate queue entries
• A moderator-only GET /moderation/queue endpoint lists pending reports sorted by report count
• Moderators can resolve a report with an action (dismiss, remove content, warn user) via PATCH /moderation/reports/:id
• Removed content is soft-deleted (retained for audit) rather than hard-deleted
• RBAC guard restricts moderation endpoints to users with a moderator or admin role