Onboarding: template backlink, bulk-create endpoint, climate-region scoring

Clients that send users through a multi-task onboarding step no longer
loop N POST /api/tasks/ calls and no longer create "orphan" tasks with
no reference to the TaskTemplate they came from.

Task model
- New task_template_id column + GORM FK (migration 000016)
- CreateTaskRequest.template_id, TaskResponse.template_id
- task_service.CreateTask persists the backlink

Bulk endpoint
- POST /api/tasks/bulk/ — 1-50 tasks in a single transaction,
  returns every created row + TotalSummary. Single residence access
  check, per-entry residence_id is overridden with batch value
- task_handler.BulkCreateTasks + task_service.BulkCreateTasks using
  db.Transaction; task_repo.CreateTx + FindByIDTx helpers

Climate-region scoring
- templateConditions gains ClimateRegionID; suggestion_service scores
  residence.PostalCode -> ZipToState -> GetClimateRegionIDByState against
  the template's conditions JSON (no penalty on mismatch / unknown ZIP)
- regionMatchBonus 0.35, totalProfileFields 14 -> 15
- Standalone GET /api/tasks/templates/by-region/ removed; legacy
  task_tasktemplate_regions many-to-many dropped (migration 000017).
  Region affinity now lives entirely in the template's conditions JSON

Tests
- +11 cases across task_service_test, task_handler_test, suggestion_
  service_test: template_id persistence, bulk rollback + cap + auth,
  region match / mismatch / no-ZIP / unknown-ZIP / stacks-with-others

Docs
- docs/openapi.yaml: /tasks/bulk/ + BulkCreateTasks schemas, template_id
  on TaskResponse + CreateTaskRequest, /templates/by-region/ removed

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/repositories/task_repo.go

@@ -355,6 +355,27 @@ func (r *TaskRepository) Create(task *models.Task) error {
 	return r.db.Create(task).Error
 }
 
+// CreateTx creates a new task within an existing transaction. Used by
+// bulk-create flows where multiple inserts must succeed or fail together.
+func (r *TaskRepository) CreateTx(tx *gorm.DB, task *models.Task) error {
+	return tx.Create(task).Error
+}
+
+// FindByIDTx loads a task within an existing transaction. Preloads only the
+// fields the bulk-create response needs (CreatedBy, AssignedTo). Category /
+// Priority / Frequency are resolved client-side from the lookup cache, so
+// we skip them here to match the FindByResidence preload set.
+func (r *TaskRepository) FindByIDTx(tx *gorm.DB, id uint) (*models.Task, error) {
+	var task models.Task
+	err := tx.Preload("CreatedBy").
+		Preload("AssignedTo").
+		First(&task, id).Error
+	if err != nil {
+		return nil, err
+	}
+	return &task, nil
+}
+
 // Update updates a task with optimistic locking.
 // The update only succeeds if the task's version in the database matches the expected version.
 // On success, the local task.Version is incremented to reflect the new version.