Add Deadline Extraction to AI Classification Output

[krishnasahoo11156]

The AI extracts action items but never extracts deadlines like “due Friday” or “submit by July 15.” This prevents the reminder scheduler from working.

Proposed Solution
Update the OpenAI structured output schema to include deadlines: string[] (ISO 8601 dates). Use chrono-node as fallback parsing.

Acceptance Criteria
• Update AI prompt schema
• Add deadlines to EmailAnalysis model
• Add deadline to ActionItem model
• Display deadlines in frontend EmailViewer
• Feed deadlines into reminder scheduler