chore: Add validation queries for default timestamp records

Added queries to identify and sample records with default timestamp
(1970-01-01 00:00:00) which resulted from invalid MySQL dates during
migration. These records need date recovery from the MySQL source.

Queries:
- Count records with default timestamp in both tables
- Sample first 10 records from rawdatacor with default timestamp

These will help quantify the scope of date recovery work needed.

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

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

scripts/validate_migration.sql

@@ -88,3 +88,23 @@ SELECT
 FROM elabdatadisp
 GROUP BY unit_name, tool_name_id, node_num, event_date, event_time
 HAVING COUNT(*) > 1;
+
+-- 10. Identify records with invalid/default timestamps (1970-01-01)
+-- These need date recovery from MySQL
+SELECT
+    'rawdatacor with default timestamp' as table_name,
+    COUNT(*) as record_count
+FROM rawdatacor
+WHERE event_timestamp = '1970-01-01 00:00:00'
+UNION ALL
+SELECT
+    'elabdatadisp with default timestamp' as table_name,
+    COUNT(*) as record_count
+FROM elabdatadisp
+WHERE event_timestamp = '1970-01-01 00:00:00';
+
+-- Sample records with default timestamp (first 10 from rawdatacor)
+SELECT id, unit_name, tool_name_id, event_timestamp, measurements
+FROM rawdatacor
+WHERE event_timestamp = '1970-01-01 00:00:00'
+LIMIT 10;