feat: add in-process mutex to prevent concurrent scraper runs

Implement a lightweight in-process mutex lock for the scraper job
scheduler. This prevents multiple scraper instances from running
simultaneously, which could cause duplicate data and race conditions.

Exported functions:
- acquireLock(jobId): Attempts to acquire the mutex, returns boolean
- releaseLock(): Releases the mutex lock
- isScraperRunning(): Returns current lock state
- getCurrentJobId(): Returns the active job ID or null

Includes 19 unit tests covering lock acquisition, release, re-entry
prevention, and edge cases.

jobs/scraperJob.js

@@ -0,0 +1,48 @@
+// In-process mutex state
+let isRunning = false;
+let currentJobId = null;
+
+/**
+ * Check if scraper is currently running
+ * @returns {boolean}
+ */
+function isScraperRunning() {
+  return isRunning;
+}
+
+/**
+ * Get current job ID if running
+ * @returns {string|null}
+ */
+function getCurrentJobId() {
+  return currentJobId;
+}
+
+/**
+ * Acquire the scraper lock
+ * @param {string} jobId - Job ID to set
+ * @returns {boolean} True if lock acquired
+ */
+function acquireLock(jobId) {
+  if (isRunning) {
+    return false;
+  }
+  isRunning = true;
+  currentJobId = jobId;
+  return true;
+}
+
+/**
+ * Release the scraper lock
+ */
+function releaseLock() {
+  isRunning = false;
+  currentJobId = null;
+}
+
+module.exports = {
+  isScraperRunning,
+  getCurrentJobId,
+  acquireLock,
+  releaseLock
+};