IndexNow integration for automatic search engine URL submission

Features

• IndexNow integration for automatic search engine URL submission
• Attachments now displayed in the email thread view
• New use case pages for billing agents and support agents

Fixes

• Rejected threads now moved to trash so they don't reappear in the inbox
• Missing @Inject decorator added to StorageController
• SEO audit — trimmed meta descriptions, added keywords to H1s, expanded cross-links
• SEO audit — fixed sitemap gaps, stale dates, title bugs, and missing JSON-LD