Implement insertPrices() bulk operation for daily price records

Add insertPrices() to scraperService that performs bulk upsert of
daily unit price records to the prices collection. Key behaviors:

- Uses bulkWrite with updateOne/upsert ops for atomic batch writes
- Compound key (unit_code + date_checked) ensures idempotent re-runs
- Filters out units with null prices before writing
- Stores price, last_updated, and data_source fields per record
- Uses ordered:false for parallel write performance
- Logs warnings for empty/all-null input without hitting the database
- Propagates bulkWrite errors with detailed logging

Includes 30 tests covering filtering, upsert behavior, composite key
logic, field validation, idempotency, error handling, and logging.

services/scraperService.js

@@ -430,11 +430,75 @@ async function upsertUnits(db, units, logger) {
   }
 }
 
+/**
+ * Insert price records for today using bulkWrite with upsert.
+ * Uses unit_code + date_checked as the composite key to prevent
+ * duplicate price records for the same unit on the same day.
+ * Re-running on the same day updates existing records (idempotent).
+ *
+ * @param {Db} db - MongoDB database instance
+ * @param {Array<Object>} units - Array of unit objects
+ * @param {string} date - Date in YYYY-MM-DD format
+ * @param {Object} logger - Logger instance
+ * @returns {Promise<Object>} Insert result with insertedCount
+ */
+async function insertPrices(db, units, date, logger) {
+  const collection = db.collection(config.COLLECTIONS.PRICES);
+  const now = new Date().toISOString();
+
+  // Filter out units without prices
+  const unitsWithPrices = units.filter(u => u.price !== null);
+
+  const priceRecords = unitsWithPrices.map(unit => ({
+    unit_code: unit.unit_code,
+    date_checked: date,
+    price: unit.price,
+    last_updated: now,
+    data_source: 'web_scraper'
+  }));
+
+  if (priceRecords.length === 0) {
+    logger.warn('No price records to insert');
+    return { insertedCount: 0 };
+  }
+
+  // Use updateOne with upsert to handle re-runs on same day
+  const operations = priceRecords.map(record => ({
+    updateOne: {
+      filter: {
+        unit_code: record.unit_code,
+        date_checked: record.date_checked
+      },
+      update: { $set: record },
+      upsert: true
+    }
+  }));
+
+  try {
+    const result = await collection.bulkWrite(operations, { ordered: false });
+
+    logger.info('Prices inserted', {
+      inserted: result.upsertedCount,
+      updated: result.modifiedCount
+    });
+
+    return { insertedCount: result.upsertedCount + result.modifiedCount };
+
+  } catch (error) {
+    logger.error('Failed to insert prices', {
+      errorType: error.name,
+      errorMessage: error.message
+    });
+    throw error;
+  }
+}
+
 module.exports = {
   fetchPage,
   parseUnits,
   convertDataTypes,
   upsertUnits,
+  insertPrices,
   // Export helpers for testing
   parseInteger,
   parsePositiveInteger,