fix: Remove unique constraint from template_type and improve AI response handling

Database Changes:
- Remove UNIQUE constraint from mail_templates.template_type
- Allow multiple templates with same type (e.g., multiple bank templates)

Ollama Service Improvements:
- Add detailed logging for AI responses (first 500 chars)
- Check multiple field name variations (subject/Subject/konu/title, body/Body/html/content)
- Improve error messages with actual field names
- Better JSON parsing with fallback
- Clearer prompt instructions to AI

This fixes:
- 400 Validation Error when creating templates with existing type
- AI response parsing issues with different field names

backend/src/models/MailTemplate.js

@@ -14,8 +14,7 @@ const MailTemplate = sequelize.define('MailTemplate', {
   template_type: {
     type: DataTypes.STRING(50),
     allowNull: false,
-    unique: true,
-    comment: 'bank, edevlet, corporate, etc.',
+    comment: 'bank, edevlet, corporate, etc. (multiple templates can have same type)',
   },
   subject_template: {
     type: DataTypes.STRING(500),