feat: minimal stabilization of migration 004 with atomic execution

- Single SQL source: Define MIG_004_SQL constant to eliminate duplicate SQL definitions
- Atomic execution: Add BEGIN IMMEDIATE/COMMIT/ROLLBACK around migration execution
- Name-only check: Skip migrations already recorded in migrations table
- Guarded operations: Replace table-wide cleanups with conditional UPDATE/DELETE

Changes:
- migration.ts: Extract migration 004 SQL into MIG_004_SQL constant
- migration.ts: Use guarded DELETE/UPDATE to prevent accidental data loss
- migrationService.ts: Wrap migration execution in explicit transactions
- migrationService.ts: Reorder checks to prioritize name-only skipping

Benefits:
- Prevents partial migration failures from corrupting database state
- Eliminates SQL duplication and maintenance overhead
- Maintains existing APIs and logging behavior
- Reduces risk of data loss during migration execution

Test results: All migration tests passing, ID generation working correctly

src/services/migrationService.ts

@@ -649,15 +649,15 @@ export async function runMigrations<T>(
       // Check 1: Is it recorded as applied in migrations table?
       const isRecordedAsApplied = appliedMigrations.has(migration.name);
 
-      // Check 2: Does the schema already exist in the database?
-      const isSchemaPresent = await isSchemaAlreadyPresent(migration, sqlQuery);
-
-      // Skip if already recorded as applied
+      // Skip if already recorded as applied (name-only check)
       if (isRecordedAsApplied) {
         skippedCount++;
         continue;
       }
 
+      // Check 2: Does the schema already exist in the database?
+      const isSchemaPresent = await isSchemaAlreadyPresent(migration, sqlQuery);
+
       // Handle case where schema exists but isn't recorded
       if (isSchemaPresent) {
         try {
@@ -687,46 +687,58 @@ export async function runMigrations<T>(
       }
 
       try {
-        // Execute the migration SQL as single atomic operation
+        // Execute the migration SQL as single atomic operation with transaction
         if (isDevelopment) {
           migrationLog(`🔧 [Migration] Executing SQL for: ${migration.name}`);
           migrationLog(`🔧 [Migration] SQL content: ${migration.sql}`);
         }
 
-        const execResult = await sqlExec(migration.sql);
+        // Begin transaction for atomic execution
+        await sqlExec("BEGIN IMMEDIATE");
+        
+        try {
+          const execResult = await sqlExec(migration.sql);
 
-        if (isDevelopment) {
-          migrationLog(
-            `🔧 [Migration] SQL execution result: ${JSON.stringify(execResult)}`,
-          );
-        }
-
-        // Validate the migration was applied correctly (only in development)
-        if (isDevelopment) {
-          const validation = await validateMigrationApplication(
-            migration,
-            sqlQuery,
-          );
-          if (!validation.isValid) {
-            logger.warn(
-              `⚠️  [Migration] Validation failed for ${migration.name}:`,
-              validation.errors,
+          if (isDevelopment) {
+            migrationLog(
+              `🔧 [Migration] SQL execution result: ${JSON.stringify(execResult)}`,
             );
           }
-        }
 
-        // Record that the migration was applied
-        await sqlExec("INSERT INTO migrations (name) VALUES (?)", [
-          migration.name,
-        ]);
+          // Validate the migration was applied correctly (only in development)
+          if (isDevelopment) {
+            const validation = await validateMigrationApplication(
+              migration,
+              sqlQuery,
+            );
+            if (!validation.isValid) {
+              logger.warn(
+                `⚠️  [Migration] Validation failed for ${migration.name}:`,
+                validation.errors,
+              );
+            }
+          }
 
-        // Only log success in development
-        if (isDevelopment) {
-          migrationLog(
-            `🎉 [Migration] Successfully applied: ${migration.name}`,
-          );
+          // Record that the migration was applied
+          await sqlExec("INSERT INTO migrations (name) VALUES (?)", [
+            migration.name,
+          ]);
+
+          // Commit transaction
+          await sqlExec("COMMIT");
+
+          // Only log success in development
+          if (isDevelopment) {
+            migrationLog(
+              `🎉 [Migration] Successfully applied: ${migration.name}`,
+            );
+          }
+          appliedCount++;
+        } catch (error) {
+          // Rollback transaction on any error
+          await sqlExec("ROLLBACK");
+          throw error;
         }
-        appliedCount++;
       } catch (error) {
         logger.error(`❌ [Migration] Error applying ${migration.name}:`, error);