feat: Implement pinned comments, locked comment threads, and a two-level reply structure with @mentions.

migration_add_pinned.sql

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+-- Migration: Add is_pinned column to comments table
+-- Migration: Add is_comments_locked column to items table
+-- Run with: psql -h <host> -U <user> -d <database> -f migration_add_pinned.sql
+
+-- Pinned comments
+ALTER TABLE comments ADD COLUMN IF NOT EXISTS is_pinned BOOLEAN DEFAULT FALSE;
+CREATE INDEX IF NOT EXISTS idx_comments_is_pinned ON comments(is_pinned) WHERE is_pinned = TRUE;
+
+-- Locked threads (prevents new comments on an item)
+ALTER TABLE items ADD COLUMN IF NOT EXISTS is_comments_locked BOOLEAN DEFAULT FALSE;