fix: prevent database migration restart failures

- Move migrations table insert inside PostgreSQL transaction for atomicity
- Add PostgreSQL error codes 42701 (duplicate column), 42710 (duplicate
  object), and 23505 (unique violation) to error handling
- Make migrations 006 and 008 idempotent with column existence checks

Fixes #107

backend/migrations/legacy/006_add_photo_counter_to_categories.js

@@ -1,4 +1,8 @@
 exports.up = async function(knex) {
+  // Check if photo_counter column already exists to make migration idempotent
+  const hasPhotoCounter = await knex.schema.hasColumn('photo_categories', 'photo_counter');
+
+  if (!hasPhotoCounter) {
     // Add photo_counter column to photo_categories table
     await knex.schema.alterTable('photo_categories', function(table) {
       table.integer('photo_counter').defaultTo(0).notNullable();
@@ -19,6 +23,7 @@ exports.up = async function(knex) {
           .update({ photo_counter: photoCount.count });
       }
     }
+  }
 };
 
 exports.down = async function(knex) {

backend/migrations/legacy/008_add_language_support_to_email_templates.js

@@ -1,12 +1,21 @@
 exports.up = async function(knex) {
-  // Add language-specific columns to email_templates
+  // Check which columns already exist to make migration idempotent
+  const hasSubjectEn = await knex.schema.hasColumn('email_templates', 'subject_en');
+  const hasSubjectDe = await knex.schema.hasColumn('email_templates', 'subject_de');
+  const hasSubjectOriginal = await knex.schema.hasColumn('email_templates', 'subject');
+
+  // Only rename columns if they haven't been renamed yet
+  if (hasSubjectOriginal && !hasSubjectEn) {
     await knex.schema.alterTable('email_templates', function(table) {
-    // Add English versions (rename existing columns for consistency)
       table.renameColumn('subject', 'subject_en');
       table.renameColumn('body_html', 'body_html_en');
       table.renameColumn('body_text', 'body_text_en');
+    });
+  }
 
-    // Add German versions
+  // Only add German columns if they don't exist
+  if (!hasSubjectDe) {
+    await knex.schema.alterTable('email_templates', function(table) {
       table.string('subject_de');
       table.text('body_html_de');
       table.text('body_text_de');
@@ -18,6 +27,7 @@ exports.up = async function(knex) {
       body_html_de: knex.raw('body_html_en'),
       body_text_de: knex.raw('body_text_en')
     });
+  }
 };
 
 exports.down = async function(knex) {

backend/migrations/run-migrations-safe.js

@@ -72,21 +72,32 @@ async function runMigrationSafely(filepath) {
       console.log(`Running migration: ${filepath}`);
 
       // Run migration in a transaction if possible
+      // IMPORTANT: Include the migrations table insert INSIDE the transaction
+      // to ensure atomicity between schema changes and tracking
       if (db.client.config.client === 'pg') {
         await db.transaction(async (trx) => {
           await migration.up(trx);
+          // Insert migration record inside transaction for atomicity
+          await trx('migrations').insert({ filename });
         });
       } else {
         await migration.up(db);
+        await db('migrations').insert({ filename });
       }
 
-      await db('migrations').insert({ filename });
       console.log(`Migration ${filepath} completed successfully`);
     }
   } catch (error) {
     // Check if error is because schema already exists
-    if (error.code === '42P07' || // PostgreSQL: relation already exists
+    // PostgreSQL error codes:
-        error.code === 'SQLITE_ERROR' && error.message.includes('already exists')) {
+    // - 42P07: duplicate_table (relation already exists)
+    // - 42701: duplicate_column (column already exists)
+    // - 42710: duplicate_object (constraint, index, etc. already exists)
+    // - 23505: unique_violation (migration record already exists)
+    const schemaExistsErrors = ['42P07', '42701', '42710', '23505'];
+    const isSQLiteAlreadyExists = error.code === 'SQLITE_ERROR' && error.message.includes('already exists');
+
+    if (schemaExistsErrors.includes(error.code) || isSQLiteAlreadyExists) {
       console.log(`Migration ${filepath} - schema already exists, marking as applied`);
       await markMigrationAsApplied(path.basename(filepath));
     } else {

backend/migrations/run-migrations.js

@@ -29,8 +29,19 @@ async function runMigration(filepath) {
 
   if (migration.up) {
     console.log(`Running migration: ${filepath}`);
+
+    // Run migration in a transaction if PostgreSQL to ensure atomicity
+    // between schema changes and migration tracking
+    if (db.client.config.client === 'pg') {
+      await db.transaction(async (trx) => {
+        await migration.up(trx);
+        await trx('migrations').insert({ filename });
+      });
+    } else {
       await migration.up(db);
       await db('migrations').insert({ filename });
+    }
+
     console.log(`Migration ${filepath} completed`);
   }
 }