fix(usage): restore /usage stats with SQLite source and JSON fallback

[paulocarinhena]

Problem

The /usage page was returning empty stats because the backend endpoint (GET /api/usage) only read legacy JSON files under storage/message and storage/session.

Recent OpenCode versions persist conversation data in opencode.db (SQLite), so environments without legacy JSON logs had no usage data to aggregate.

Solution

• Added native SQLite support using Node's built-in node:sqlite (DatabaseSync) in the usage endpoint.
• Implemented a SQLite-first read path that aggregates usage from message, session, and project tables.
• Preserved the existing response contract (totalCost, totalTokens, byModel, byDay, byProject) so no frontend changes are required.
• Kept backwards compatibility via fallback to the previous JSON-based parser when SQLite is unavailable or fails.
• Kept all existing filters and grouping behavior (projectId, range, from, to, granularity), including per-model timeline token keys used by charts.

Why this works

The endpoint now supports both storage formats used across OpenCode versions. New installs get data from SQLite immediately, while older installs continue to work with JSON logs.

Validation

• node --check server/index.js
• Manual API path verification for /api/usage with standard filters/ranges.

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