[Analytics] Add platform-level time series endpoint

[PierreLeGuen]

Description

Add a new endpoint to provide platform-wide time series data for charting trends.

Motivation

The admin dashboard currently shows platform summary metrics, but lacks time series charts showing trends over time (requests, revenue, tokens).

Proposed Solution

Add GET /v1/admin/platform/metrics/timeseries endpoint that returns:

• Requests over time
• Revenue over time
• Token usage over time (input/output)
• Support for granularity (hour/day/week)

Technical Details

• Similar to /v1/admin/organizations/{org_id}/metrics/timeseries but aggregated across all orgs
• Support time range filtering
• Support granularity parameter

Acceptance Criteria

• New endpoint returns time series data
• Supports granularity (hour/day/week)
• Supports time range filtering
• Returns requests, revenue, and token counts per time bucket