Fix duplicate key error in migration marking

- Added check in markMigrationAsApplied to prevent duplicate inserts
- Now checks if migration is already marked before inserting
- Prevents 'duplicate key value violates unique constraint' error

The error occurred when detectExistingSchema() marked a migration
as applied, then the migration runner caught a 'schema exists' error
and tried to mark it as applied again.

backend/migrations/run-migrations-safe.js

@@ -27,8 +27,12 @@ async function isMigrationApplied(filename) {
 
 // Mark migration as applied without running it (for existing schema)
 async function markMigrationAsApplied(filename) {
-  await db('migrations').insert({ filename });
-  console.log(`Marked migration ${filename} as applied`);
+  // Check if already marked to avoid duplicate key error
+  const isApplied = await isMigrationApplied(filename);
+  if (!isApplied) {
+    await db('migrations').insert({ filename });
+    console.log(`Marked migration ${filename} as applied`);
+  }
 }
 
 // Detect existing schema and mark migrations as applied