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/article_generation_queue.go

@@ -0,0 +1,63 @@
+package app
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	"github.com/geo-platform/tenant-api/internal/shared/messaging/rabbitmq"
+)
+
+type articleGenerationTaskEnvelope struct {
+	TaskID    int64  `json:"task_id"`
+	TaskType  string `json:"task_type"`
+	TenantID  int64  `json:"tenant_id"`
+	ArticleID int64  `json:"article_id"`
+}
+
+type ArticleGenerationTaskEnvelope struct {
+	TaskID    int64  `json:"task_id"`
+	TaskType  string `json:"task_type"`
+	TenantID  int64  `json:"tenant_id"`
+	ArticleID int64  `json:"article_id"`
+}
+
+func publishArticleGenerationTask(ctx context.Context, client *rabbitmq.Client, envelope articleGenerationTaskEnvelope) error {
+	if client == nil {
+		return fmt.Errorf("rabbitmq client is not configured")
+	}
+
+	payload, err := json.Marshal(envelope)
+	if err != nil {
+		return fmt.Errorf("marshal generation task envelope: %w", err)
+	}
+
+	if err := client.PublishGenerationTask(ctx, payload); err != nil {
+		return fmt.Errorf("publish generation task: %w", err)
+	}
+	return nil
+}
+
+func decodeArticleGenerationTaskEnvelope(payload []byte) (*articleGenerationTaskEnvelope, error) {
+	var envelope articleGenerationTaskEnvelope
+	if err := json.Unmarshal(payload, &envelope); err != nil {
+		return nil, fmt.Errorf("decode generation task envelope: %w", err)
+	}
+	if envelope.TaskID <= 0 {
+		return nil, fmt.Errorf("generation task envelope task_id is required")
+	}
+	return &envelope, nil
+}
+
+func DecodeArticleGenerationTaskEnvelope(payload []byte) (*ArticleGenerationTaskEnvelope, error) {
+	envelope, err := decodeArticleGenerationTaskEnvelope(payload)
+	if err != nil {
+		return nil, err
+	}
+	return &ArticleGenerationTaskEnvelope{
+		TaskID:    envelope.TaskID,
+		TaskType:  envelope.TaskType,
+		TenantID:  envelope.TenantID,
+		ArticleID: envelope.ArticleID,
+	}, nil
+}