feat(admin-dash): P1 — sparklines, content inventory, subject distribution, worst-5 sessions
- 7d sparkline per 3 main metric cards (inline SVG polyline, renderSparkline helper)
- "Контент проекта" row: questions/tests/courses/classes totals (compact .ov-inv-grid)
- Per-subject stacked bar (24h) with hue-cycle colors and legend below
- "Худшие 5 сегодня" mirrors top-5 table; both side-by-side ≥1100px via .ov-results-grid
- renderSessionRows() shared helper for top/worst table rows
Backend: 5 new prepared statements (worstSessions24h, sparkUsers7d, sparkSessions7d,
sparkActiveUsers7d, inventory, sessionsBySubject24h)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Maxim Dolgolyov
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