Implement /api/ai/retro-summary endpoint

[romahawk]

Part of #60

What

Build the API route that generates a retrospective draft from the week's execution data.

Tasks

• Create app/api/ai/retro-summary/route.ts
• Input: WeeklyPlan, ExecutionLogs, completed tasks, journal entries from the current week
• Output: { wins: string[], friction: string[], nextWeekIntentions: string[], summary: string }
• Keep response under 400 tokens (concise, actionable)
• Cache per user per week; do not regenerate if user has already saved edits

AC

• Returns structured JSON matching the Journal section schema
• Cached response returned if week's data hasn't changed significantly