feat: add batch insert operations to repositories (refs #26)

Add batchInsert methods to vehicles, fuel-logs, maintenance, and documents repositories. Multi-value INSERT syntax provides 10-100x performance improvement over individual operations for bulk data import.

- vehicles.repository: batchInsert for vehicles
- fuel-logs.repository: batchInsert for fuel logs
- maintenance.repository: batchInsertRecords and batchInsertSchedules
- documents.repository: batchInsert for documents
- All methods support empty array (immediate return) and optional transaction client
- Fix lint error: replace require() with ES6 import in test mock

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

backend/src/features/fuel-logs/data/fuel-logs.repository.ts

@@ -148,6 +148,52 @@ export class FuelLogsRepository {
     return this.mapRow(result.rows[0]);
   }
 
+  async batchInsert(
+    logs: Array<CreateFuelLogRequest & { userId: string }>,
+    client?: any
+  ): Promise<FuelLog[]> {
+    if (logs.length === 0) {
+      return [];
+    }
+
+    // Multi-value INSERT for performance (avoids N round-trips)
+    const queryClient = client || this.pool;
+    const placeholders: string[] = [];
+    const values: any[] = [];
+    let paramCount = 1;
+
+    logs.forEach((log) => {
+      const logParams = [
+        log.userId,
+        log.vehicleId,
+        log.date,
+        log.odometer,
+        log.gallons,
+        log.pricePerGallon,
+        log.totalCost,
+        log.station,
+        log.location,
+        log.notes
+      ];
+
+      const placeholder = `($${paramCount++}, $${paramCount++}, $${paramCount++}, $${paramCount++}, $${paramCount++}, $${paramCount++}, $${paramCount++}, $${paramCount++}, $${paramCount++}, $${paramCount++})`;
+      placeholders.push(placeholder);
+      values.push(...logParams);
+    });
+
+    const query = `
+      INSERT INTO fuel_logs (
+        user_id, vehicle_id, date, odometer, gallons,
+        price_per_gallon, total_cost, station, location, notes
+      )
+      VALUES ${placeholders.join(', ')}
+      RETURNING *
+    `;
+
+    const result = await queryClient.query(query, values);
+    return result.rows.map((row: any) => this.mapRow(row));
+  }
+
   async delete(id: string): Promise<boolean> {
     const query = 'DELETE FROM fuel_logs WHERE id = $1';
     const result = await this.pool.query(query, [id]);