feat(classroom): add active sessions API endpoint

Provides API for useActiveSessionsInClassroom hook to fetch
which students in a classroom are currently practicing.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/src/app/api/classrooms/[classroomId]/presence/active-sessions/route.ts

@@ -0,0 +1,119 @@
+import { and, eq, inArray, isNull } from 'drizzle-orm'
+import { type NextRequest, NextResponse } from 'next/server'
+import { db, schema } from '@/db'
+import { getClassroomPresence, getTeacherClassroom } from '@/lib/classroom'
+import { getViewerId } from '@/lib/viewer'
+
+/**
+ * Get or create user record for a viewerId (guestId)
+ */
+async function getOrCreateUser(viewerId: string) {
+  let user = await db.query.users.findFirst({
+    where: eq(schema.users.guestId, viewerId),
+  })
+
+  if (!user) {
+    const [newUser] = await db.insert(schema.users).values({ guestId: viewerId }).returning()
+    user = newUser
+  }
+
+  return user
+}
+
+interface RouteParams {
+  params: Promise<{ classroomId: string }>
+}
+
+/**
+ * Active session information returned by this endpoint
+ */
+interface ActiveSessionInfo {
+  /** Session plan ID (for observation) */
+  sessionId: string
+  /** Player ID */
+  playerId: string
+  /** When the session started */
+  startedAt: Date
+  /** Current part index */
+  currentPartIndex: number
+  /** Current slot index within the part */
+  currentSlotIndex: number
+  /** Total parts in session */
+  totalParts: number
+  /** Total problems in session (sum of all slots) */
+  totalProblems: number
+  /** Number of completed problems */
+  completedProblems: number
+}
+
+/**
+ * GET /api/classrooms/[classroomId]/presence/active-sessions
+ * Get active practice sessions for students currently present in the classroom
+ *
+ * Returns: { sessions: ActiveSessionInfo[] }
+ *
+ * This endpoint allows teachers to see which students are actively practicing
+ * so they can observe their sessions in real-time.
+ */
+export async function GET(req: NextRequest, { params }: RouteParams) {
+  try {
+    const { classroomId } = await params
+    const viewerId = await getViewerId()
+    const user = await getOrCreateUser(viewerId)
+
+    // Verify user is the teacher of this classroom
+    const classroom = await getTeacherClassroom(user.id)
+    if (!classroom || classroom.id !== classroomId) {
+      return NextResponse.json({ error: 'Not authorized' }, { status: 403 })
+    }
+
+    // Get all students currently present in the classroom
+    const presences = await getClassroomPresence(classroomId)
+    // Filter out presences where player was deleted (undefined)
+    const playerIds = presences.filter((p) => p.player !== undefined).map((p) => p.player!.id)
+
+    if (playerIds.length === 0) {
+      return NextResponse.json({ sessions: [] })
+    }
+
+    // Find active sessions for these players
+    // Active = status is 'in_progress', startedAt is set, completedAt is null
+    const activeSessions = await db.query.sessionPlans.findMany({
+      where: and(
+        inArray(schema.sessionPlans.playerId, playerIds),
+        eq(schema.sessionPlans.status, 'in_progress'),
+        isNull(schema.sessionPlans.completedAt)
+      ),
+    })
+
+    // Map to ActiveSessionInfo
+    const sessions: ActiveSessionInfo[] = activeSessions
+      .filter((session) => session.startedAt)
+      .map((session) => {
+        // Calculate total and completed problems
+        const parts = session.parts
+        const totalProblems = parts.reduce((sum, part) => sum + part.slots.length, 0)
+        let completedProblems = 0
+        for (let i = 0; i < session.currentPartIndex; i++) {
+          completedProblems += parts[i].slots.length
+        }
+        completedProblems += session.currentSlotIndex
+
+        return {
+          sessionId: session.id,
+          playerId: session.playerId,
+          startedAt: session.startedAt as Date,
+          currentPartIndex: session.currentPartIndex,
+          currentSlotIndex: session.currentSlotIndex,
+          totalParts: parts.length,
+          totalProblems,
+          completedProblems,
+        }
+      })
+
+    return NextResponse.json({ sessions })
+  } catch (error) {
+    console.error('Failed to fetch active sessions:', error)
+    return NextResponse.json({ error: 'Failed to fetch active sessions' }, { status: 500 })
+  }
+}