feat: add token usage and cost dashboard - #347

Overview

This PR adds a token usage and cost dashboard at /usage, showing the current user's total tokens and estimated cost for the current month, plus a daily usage chart for the last 30 days. Data is sourced from the existing token usage tracking service, and the page is protected by @login_required so each user only sees their own data.

Related Issue

Changes

📊 Usage Dashboard

• [ADD] app/templates/main/usage.html

  • New /usage page with summary cards for total tokens and estimated cost for the current month.
  • Includes a chart container for the last 30 days of daily usage.

• [ADD] app/static/js/usage.js

  • Fetches the JSON usage summary endpoint and renders the daily usage chart plus summary values.

• [MODIFY] app/templates/base.html

  • Adds navigation entry for the new usage dashboard.

🧮 Usage Aggregation

• [MODIFY] app/services/token_usage.py

  • Adds aggregation helpers to compute current-month totals and per-day usage for the last 30 days from tracked token usage records.

• [MODIFY] app/main/routes.py

  • Adds the @login_required /usage page route scoped to the current user.
  • Adds a JSON usage summary endpoint returning totals and daily series for the current user.

✅ Tests

• [MODIFY] tests/test_token_usage.py
  • Adds coverage for the /usage page render and the JSON usage summary endpoint.

Verification Results

pytest tests/test_token_usage.py
✅ passed

Closes #32