feat: add natural language finance query endpoint

[Mohye24k]

Summary

• Add POST /query endpoint that accepts natural language questions about finances
• Parse questions using regex patterns to match: "how much spent", "spent on X", "income this month", "biggest expense"
• Return structured answer with human-readable text and data payload
• Graceful fallback with suggestions for unrecognized queries
• Add comprehensive test suite (auth, how much query, unknown query, category query)
• Add TypeScript API client (app/src/api/nlquery.ts)
• Register blueprint in __init__.py

Test plan

• Auth required returns 401
• "how much did I spend this month?" returns correct total
• "how much did I spend on food?" matches category and returns total
• Unknown questions return helpful suggestions
• Empty question returns 400

/claim #74
Fixes #74