feat: add student conflict detection in appointments

- Add hasStudentConflict() to Appointment model
- Check student conflicts in store() and update()
- Returns 409 Conflict when student has overlapping appointment
- Fixed PostgreSQL sequence for appointments id

api/src/Controllers/AppointmentsController.php

@@ -42,6 +42,19 @@ final class AppointmentsController
             }
         }
 
+        // Conflict check for student
+        if (isset($data['student_id'], $data['start_at'], $data['end_at'])) {
+            if (Appointment::hasStudentConflict(
+                $data['student_id'] ? (int) $data['student_id'] : null,
+                $data['start_at'],
+                $data['end_at']
+            )) {
+                return $this->json($response, [
+                    'message' => 'Student has a conflicting appointment in this time range'
+                ], 409);
+            }
+        }
+
         $appointment = Appointment::create([
             'tenant_id' => $tenantId,
             'student_id' => $data['student_id'] ?? null,
@@ -84,6 +97,20 @@ final class AppointmentsController
             }
         }
 
+        // Conflict check for student (exclude current appointment)
+        if (isset($data['student_id'], $data['start_at'], $data['end_at'])) {
+            if (Appointment::hasStudentConflict(
+                $data['student_id'] ? (int) $data['student_id'] : null,
+                $data['start_at'],
+                $data['end_at'],
+                (int) $args['id']
+            )) {
+                return $this->json($response, [
+                    'message' => 'Student has a conflicting appointment in this time range'
+                ], 409);
+            }
+        }
+
         $data['updated_by'] = $user['id'];
         $appointment->update(array_intersect_key($data, array_flip([
             'student_id', 'instructor_id', 'lesson_type_id', 'title', 'start_at', 'end_at', 'units', 'status', 'notes', 'warning_acknowledged', 'updated_by'

api/src/Models/Appointment.php

@@ -28,7 +28,6 @@ final class Appointment extends Model
 
     /**
      * Check if instructor has a conflicting appointment in the given time range.
-     * Excludes the appointment with $excludeId (for updates).
      */
     public static function hasInstructorConflict(
         int $instructorId,
@@ -52,4 +51,34 @@ final class Appointment extends Model
 
         return $query->exists();
     }
+
+    /**
+     * Check if student has a conflicting appointment in the given time range.
+     */
+    public static function hasStudentConflict(
+        ?int $studentId,
+        string $start,
+        string $end,
+        ?int $excludeId = null
+    ): bool {
+        if ($studentId === null) {
+            return false; // No student assigned, no conflict possible
+        }
+
+        $query = self::where('student_id', $studentId)
+            ->where('status', '!=', 'cancelled')
+            ->where(function ($q) use ($start, $end) {
+                $q->whereBetween('start_at', [$start, $end])
+                  ->orWhereBetween('end_at', [$start, $end])
+                  ->orWhere(function ($q2) use ($start, $end) {
+                      $q2->where('start_at', '<=', $start)->where('end_at', '>=', $end);
+                  });
+            });
+
+        if ($excludeId !== null) {
+            $query->where('id', '!=', $excludeId);
+        }
+
+        return $query->exists();
+    }
 }