feat(generation): migrate article generation and template-assist to RabbitMQ queues

Replace in-process generation with queue-based async execution via
RabbitMQ. Add generation task runtime for lifecycle management, article
generation payload/queue abstractions, and template-assist queue publisher.
Refactor prompt generation service to support batch generation with
per-item error tracking. Update stream hub to bridge queue events to SSE.

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

server/internal/tenant/app/template_assist_queue.go

@@ -0,0 +1,44 @@
+package app
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	"github.com/geo-platform/tenant-api/internal/shared/messaging/rabbitmq"
+)
+
+func publishTemplateAssistTask(ctx context.Context, client *rabbitmq.Client, job assistJob) error {
+	if client == nil {
+		return fmt.Errorf("rabbitmq client is not configured")
+	}
+
+	payload, err := json.Marshal(job)
+	if err != nil {
+		return fmt.Errorf("marshal template assist job: %w", err)
+	}
+
+	if err := client.PublishTemplateAssistTask(ctx, payload); err != nil {
+		return fmt.Errorf("publish template assist task: %w", err)
+	}
+	return nil
+}
+
+func decodeTemplateAssistTask(payload []byte) (*assistJob, error) {
+	var job assistJob
+	if err := json.Unmarshal(payload, &job); err != nil {
+		return nil, fmt.Errorf("decode template assist job: %w", err)
+	}
+	if job.TaskID == "" || job.TaskType == "" || job.TenantID <= 0 || job.TemplateID <= 0 {
+		return nil, fmt.Errorf("template assist job is incomplete")
+	}
+	return &job, nil
+}
+
+func DecodeTemplateAssistTask(payload []byte) (*AssistJob, error) {
+	job, err := decodeTemplateAssistTask(payload)
+	if err != nil {
+		return nil, err
+	}
+	return job, nil
+}