feat: Add publish and mark-published endpoints with validation

- Add /api/posts/{id}/publish endpoint for API-based publishing
- Add /api/posts/{id}/mark-published endpoint for manual workflow
- Add content length validation before publishing
- Update modal with "Ya lo publiqué" and "Publicar (API)" buttons
- Fix retry_count handling for None values

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

worker/tasks/publish_post.py

@@ -99,6 +99,18 @@ def publish_to_platform(self, post_id: int, platform: str):
         else:
             # Publicación normal
             content = post.get_content_for_platform(platform)
+
+            # Validar longitud antes de publicar
+            if hasattr(publisher, 'char_limit') and len(content) > publisher.char_limit:
+                error_msg = (
+                    f"Contenido excede límite: {len(content)}/{publisher.char_limit} "
+                    f"caracteres (sobran {len(content) - publisher.char_limit})"
+                )
+                post.error_message = f"{platform}: {error_msg}"
+                post.status = "failed"
+                db.commit()
+                return f"Error en {platform}: {error_msg}"
+
             result = run_async(
                 publisher.publish(content, post.image_url)
             )
@@ -125,7 +137,7 @@ def publish_to_platform(self, post_id: int, platform: str):
         else:
             # Error en publicación
             post.error_message = f"{platform}: {result.error_message}"
-            post.retry_count += 1
+            post.retry_count = (post.retry_count or 0) + 1
 
             if post.retry_count >= 3:
                 post.status = "failed"