feat: Add natural language query endpoint for financial data

[Elvis-scholar]

Implements Issue #74 - Natural Language Finance Query

Features:

• New /query endpoint that accepts natural language queries
• Supports date range parsing (last quarter, this month, last year, etc.)
• Automatic category extraction from query text
• Returns accurate answer with source data
• Optional Gemini AI integration for better answer formatting
• Comprehensive test coverage

Example queries:

• 'How much did I spend on food last quarter?'
• 'What did I spend this month?'
• 'How much did I spend in the last 30 days?'

Technical details:

• Added nlp_query.py service with date parsing and category matching
• Added query.py route with JWT authentication
• Added comprehensive pytest tests
• Handles edge cases (empty query, missing auth, etc.)

Closes #74

Summary

• What changed:
• Why:

Validation

• Frontend lint: cd app && npm run lint
• Frontend tests: cd app && npm test -- --runInBand
• Backend tests: ./scripts/test-backend.ps1
• Updated docs if needed

Security and Ownership

• PR opened from a fork (not direct push to main)
• CODEOWNERS review requested

Checklist

• No secrets added
• No unrelated files changed
• Breaking changes documented

[Elvis-scholar]

Hi @rohitdash08,

This PR implements Issue #74 which is labeled as "bounty" and "high impact".
Could you please confirm the bounty amount and payment method?

Thanks!