Add task templates API and admin management

- Add TaskTemplate model with category and frequency support
- Add task template repository with CRUD and search operations
- Add task template service layer
- Add public API endpoints for templates (no auth required):
  - GET /api/tasks/templates/ - list all templates
  - GET /api/tasks/templates/grouped/ - templates grouped by category
  - GET /api/tasks/templates/search/?q= - search templates
  - GET /api/tasks/templates/by-category/:id/ - templates by category
  - GET /api/tasks/templates/:id/ - single template
- Add admin panel for task template management (CRUD)
- Add admin API endpoints for templates
- Add seed file with predefined task templates
- Add i18n translations for template errors

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

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

internal/i18n/translations/en.json

@@ -114,6 +114,9 @@
   "error.failed_to_fetch_task_frequencies": "Failed to fetch task frequencies",
   "error.failed_to_fetch_task_statuses": "Failed to fetch task statuses",
   "error.failed_to_fetch_contractor_specialties": "Failed to fetch contractor specialties",
+  "error.failed_to_fetch_templates": "Failed to fetch task templates",
+  "error.failed_to_search_templates": "Failed to search task templates",
+  "error.template_not_found": "Task template not found",
 
   "push.task_due_soon.title": "Task Due Soon",
   "push.task_due_soon.body": "{{.TaskTitle}} is due {{.DueDate}}",