Return time-series quality metrics for trend visualization.

Buckets quality data by day (default) or week and returns per-bucket success rates, gate pass rates, and average quality scores. Covers the last 90 days by default so dashboards can show weeks-to-months trends.

Query parameters:

• days: lookback window in days (default 90, max 365).
• granularity: "day" (default) or "week".

Returns a series list ordered by date, each entry containing:

• date: ISO date string (bucket start).
• ts: Unix timestamp of the bucket start.
• tasks_total, tasks_success: raw task counts.
• success_rate: fraction of tasks that succeeded (omitted if no tasks).
• gate_pass_rates: dict of gate name → pass rate for that bucket.
• avg_quality_score: mean quality score 0-100 (omitted if no scores).