fix(entry-point-discovery): Self-healing duplicate detection

When resolving platform_dispensary_id, check if it already exists on
another dispensary. If so, mark current dispensary as duplicate instead
of failing with unique constraint violation.

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

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

backend/src/tasks/handlers/entry-point-discovery.ts

@@ -501,6 +501,35 @@ async function updateDispensaryWithPlatformId(
   source: string,
   slug: string
 ): Promise<void> {
+  // Self-healing: Check if platform_dispensary_id already exists on another dispensary
+  const existingResult = await pool.query(`
+    SELECT id, name FROM dispensaries
+    WHERE platform_dispensary_id = $1 AND id != $2
+  `, [platformId, dispensaryId]);
+
+  if (existingResult.rows.length > 0) {
+    const existing = existingResult.rows[0];
+    console.log(`[EntryPointDiscovery] Platform ID ${platformId} already exists on dispensary ${existing.id} (${existing.name})`);
+    console.log(`[EntryPointDiscovery] Marking dispensary ${dispensaryId} as duplicate of ${existing.id}`);
+
+    // Mark current dispensary as duplicate and disable it
+    await pool.query(`
+      UPDATE dispensaries
+      SET
+        id_resolution_status = 'duplicate',
+        id_resolution_error = $2,
+        crawl_enabled = false,
+        stage = 'duplicate',
+        updated_at = NOW(),
+        last_modified_at = NOW(),
+        last_modified_by_task = $3,
+        last_modified_task_id = $4
+      WHERE id = $1
+    `, [dispensaryId, `Duplicate of dispensary ${existing.id} (${existing.name})`, task.role, task.id]);
+
+    return; // Don't queue product_discovery for duplicates
+  }
+
   // Update dispensary with platform ID and stage checkpoint
   // Stage transitions: discovered/validated → promoted (ready for crawling)
   await pool.query(`