fix(pricing): book daily cost to the day tokens were spent

token_usage holds a session's cumulative totals with no time, so
/api/pricing/cost booked a multi-day session's whole cost to its start
date and the status bar's Today read ~$0. TranscriptCache now also splits
usage by each message's UTC hour (tokensByHour); the hook writer stores it
in the new token_usage_hourly table (MAX per hour) and the server
back-fills existing sessions at start. The daily split books each total
over its hourly rows' local days and the rest to the start date, so the
days still add up to the total.

docs/API.md

@@ -1864,6 +1864,18 @@ curl -X DELETE http://localhost:4820/api/pricing/gpt-5.1-codex
 | 403 | Cannot delete default rule |
 | 500 | Database error |
 
+#### Total Cost
+
+```http
+GET /api/pricing/cost?tz_offset=<minutes>&sources=<csv>
+```
+
+The total cost with a per-model `breakdown`, plus `daily_costs: [{date, cost}]` by local date (`tz_offset` is the browser's `getTimezoneOffset()`; the status bar's **Today** is the entry for today). A day is the day the tokens were SPENT: each session's total is split over the local days of its `token_usage_hourly` rows (one per UTC hour, from each Claude transcript message's timestamp), and whatever the total holds beyond those rows — a session recorded before that table and whose transcript is gone, sub-agent files, Codex and Antigravity sessions — is booked to the session's start date. The days always add up to the total.
+
+```bash
+curl "http://localhost:4820/api/pricing/cost?tz_offset=-420"
+```
+
 ---
 
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