fix: Use timezone-aware timestamps for migration state tracking

Fix timezone inconsistency between migration_started_at and migration_completed_at:

Schema Changes:
- Change TIMESTAMP to TIMESTAMPTZ for migration_started_at and migration_completed_at
- PostgreSQL stores timestamps in UTC and converts to local timezone on display
- Ensures consistent timezone handling across all timestamp columns

Code Changes:
- Replace datetime.utcnow() with datetime.now(timezone.utc)
- Use timezone-aware datetime objects for proper TIMESTAMPTZ handling
- Import timezone module for UTC timezone support

Impact:
- Previous issue: migration_completed_at was 1 hour behind migration_started_at
- Root cause: CURRENT_TIMESTAMP (local time) vs datetime.utcnow() (UTC naive)
- Solution: Both columns now use TIMESTAMPTZ with timezone-aware datetimes

Note: Existing migration_state records will have old TIMESTAMP format until table is altered:
  ALTER TABLE migration_state
    ALTER COLUMN migration_started_at TYPE TIMESTAMPTZ,
    ALTER COLUMN migration_completed_at TYPE TIMESTAMPTZ;

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

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

src/migrator/state_manager.py

@@ -7,7 +7,7 @@ Tracks migration progress with:
 - status: pending, in_progress, completed
 """
 from typing import Optional, Dict, Any
-from datetime import datetime
+from datetime import datetime, timezone
 import json
 
 from src.connectors.postgres_connector import PostgreSQLConnector
@@ -199,7 +199,7 @@ class StateManager:
 
                 if mark_completed:
                     updates.append("migration_completed_at = %s")
-                    params.append(datetime.utcnow())
+                    params.append(datetime.now(timezone.utc))
                     if status is None:
                         updates.append("status = 'completed'")
 

src/transformers/schema_transformer.py

@@ -183,8 +183,8 @@ CREATE TABLE IF NOT EXISTS migration_state (
     table_name VARCHAR(255) NOT NULL,
     partition_name VARCHAR(255) NOT NULL,
     last_key JSONB,
-    migration_started_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+    migration_started_at TIMESTAMPTZ DEFAULT CURRENT_TIMESTAMP,
-    migration_completed_at TIMESTAMP,
+    migration_completed_at TIMESTAMPTZ,
     total_rows_migrated BIGINT DEFAULT 0,
     status VARCHAR(32) DEFAULT 'pending',
     PRIMARY KEY (table_name, partition_name),