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>

backend/src/controllers/adminController.js

@@ -73,6 +73,48 @@ const overviewStmts = {
     ORDER BY (CAST(ts.score AS REAL) / ts.total) DESC, ts.finished_at DESC
     LIMIT 5
   `),
+  worstSessions24h: db.prepare(`
+    SELECT ts.id, u.name AS user_name, s.name AS subject_name,
+           ts.score, ts.total,
+           ROUND(CAST(ts.score AS REAL) / ts.total * 100, 1) AS percent,
+           ts.finished_at
+    FROM test_sessions ts
+    JOIN users u    ON u.id = ts.user_id
+    LEFT JOIN subjects s ON s.id = ts.subject_id
+    WHERE ts.status = 'completed'
+      AND ts.finished_at >= datetime('now', '-24 hours')
+      AND ts.total > 0
+    ORDER BY (CAST(ts.score AS REAL) / ts.total) ASC, ts.finished_at DESC
+    LIMIT 5
+  `),
+  sparkUsers7d: db.prepare(`
+    SELECT date(created_at) AS d, COUNT(*) AS n FROM users
+    WHERE created_at >= date('now', '-6 days')
+    GROUP BY d ORDER BY d
+  `),
+  sparkSessions7d: db.prepare(`
+    SELECT date(started_at) AS d, COUNT(*) AS n FROM test_sessions
+    WHERE started_at >= date('now', '-6 days')
+    GROUP BY d ORDER BY d
+  `),
+  sparkActiveUsers7d: db.prepare(`
+    SELECT date(last_login) AS d, COUNT(DISTINCT id) AS n FROM users
+    WHERE last_login IS NOT NULL AND last_login >= date('now', '-6 days')
+    GROUP BY d ORDER BY d
+  `),
+  inventory: db.prepare(`
+    SELECT
+      (SELECT COUNT(*) FROM questions) AS questions,
+      (SELECT COUNT(*) FROM tests)     AS tests,
+      (SELECT COUNT(*) FROM courses)   AS courses,
+      (SELECT COUNT(*) FROM classes)   AS classes
+  `),
+  sessionsBySubject24h: db.prepare(`
+    SELECT s.slug, s.name, COUNT(*) AS n FROM test_sessions ts
+    JOIN subjects s ON s.id = ts.subject_id
+    WHERE ts.started_at >= datetime('now', '-24 hours')
+    GROUP BY s.id ORDER BY n DESC
+  `),
 };
 
 /* ── GET /api/admin/overview ──────────────────────────────────────────── */
@@ -87,6 +129,14 @@ function getOverview(_req, res) {
       stuckSessions:         overviewStmts.stuckSessions.all(),
       bannedThisWeek:        overviewStmts.bannedThisWeek.all(),
       topSessions24h:        overviewStmts.topSessions24h.all(),
+      worstSessions24h:      overviewStmts.worstSessions24h.all(),
+      inventory:             overviewStmts.inventory.get(),
+      sessionsBySubject24h:  overviewStmts.sessionsBySubject24h.all(),
+      sparks: {
+        users:    overviewStmts.sparkUsers7d.all(),
+        sessions: overviewStmts.sparkSessions7d.all(),
+        active:   overviewStmts.sparkActiveUsers7d.all(),
+      },
     });
   } catch (err) {
     res.status(500).json({ error: err.message });