Add post context to content-renderer sanitization warnings

Closes #763

Summary

• Include author and permlink in renderer warning messages to identify which posts trigger blocked iframe/image warnings
• Previously warnings like "Blocked image (invalid src)" gave no indication of which post content caused the issue
• Now warnings include context like Blocked iframe (not whitelisted) in @author/permlink: src="..."

Changes

• Add PostContext interface with optional author/permlink fields
• Update TagTransformingSanitizer.sanitize() to accept and use context
• Update warning messages to include formatPostContext() output
• Export PostContext type from renderer package
• Update DefaultRenderer.render() signature to pass context
• Update RendererContainer component to accept/pass permlink prop
• Update content.tsx and comment-list-item.tsx call sites

Test plan

• Deploy and check logs for warnings - should now include post context
• Verify posts with invalid images show warning with @author/permlink
• Verify posts with non-whitelisted iframes show warning with context