docs(developer): add celery to testing section for alerts&reports (#38888)

docs/developer_docs/testing/backend-testing.md

@@ -63,6 +63,109 @@ pytest tests/unit_tests/
 pytest tests/integration_tests/
 ```
 
+## Testing Alerts & Reports with Celery and MailHog
+
+The Alerts & Reports feature relies on Celery for task scheduling and execution. To test it locally, you need Redis (message broker), Celery Beat (scheduler), a Celery Worker (executor), and an SMTP server to receive email notifications.
+
+### Prerequisites
+
+- Redis running on `localhost:6379`
+- [MailHog](https://github.com/mailhog/MailHog) installed (a local SMTP server with a web UI for viewing caught emails)
+
+### superset_config.py
+
+Your `CeleryConfig` **must** include `beat_schedule`. When you define a custom `CeleryConfig` class in `superset_config.py`, it replaces the default entirely. If you omit `beat_schedule`, Celery Beat will start but never schedule any report tasks.
+
+```python
+from celery.schedules import crontab
+from superset.tasks.types import ExecutorType
+
+REDIS_HOST = "localhost"
+REDIS_PORT = "6379"
+
+class CeleryConfig:
+    broker_url = f"redis://{REDIS_HOST}:{REDIS_PORT}/0"
+    result_backend = f"redis://{REDIS_HOST}:{REDIS_PORT}/0"
+    broker_connection_retry_on_startup = True
+    imports = (
+        "superset.sql_lab",
+        "superset.tasks.scheduler",
+        "superset.tasks.thumbnails",
+        "superset.tasks.cache",
+    )
+    worker_prefetch_multiplier = 10
+    task_acks_late = True
+    beat_schedule = {
+        "reports.scheduler": {
+            "task": "reports.scheduler",
+            "schedule": crontab(minute="*", hour="*"),
+        },
+        "reports.prune_log": {
+            "task": "reports.prune_log",
+            "schedule": crontab(minute=0, hour=0),
+        },
+    }
+
+CELERY_CONFIG = CeleryConfig
+
+# SMTP settings pointing to MailHog
+SMTP_HOST = "localhost"
+SMTP_PORT = 1025
+SMTP_STARTTLS = False
+SMTP_SSL = False
+SMTP_USER = ""
+SMTP_PASSWORD = ""
+SMTP_MAIL_FROM = "superset@localhost"
+
+# Must match where your frontend is running
+WEBDRIVER_BASEURL = "http://localhost:9000/"
+
+ALERT_REPORTS_EXECUTE_AS = [ExecutorType.OWNER]
+
+FEATURE_FLAGS = {
+    "ALERT_REPORTS": True,
+    # Recommended for better screenshot support (WebGL/DeckGL charts)
+    "PLAYWRIGHT_REPORTS_AND_THUMBNAILS": True,
+}
+```
+
+:::note
+Do not include `"superset.tasks.async_queries"` in `CeleryConfig.imports` unless you need Global Async Queries. That module accesses `current_app.config` at import time and will crash the worker with a "Working outside of application context" error.
+:::
+
+### Starting the Services
+
+Start MailHog, then Celery Beat and Worker in separate terminals:
+
+```bash
+# Terminal 1 - MailHog (SMTP on :1025, Web UI on :8025)
+MailHog
+
+# Terminal 2 - Celery Beat (scheduler)
+celery --app=superset.tasks.celery_app:app beat --loglevel=info
+
+# Terminal 3 - Celery Worker (executor)
+celery --app=superset.tasks.celery_app:app worker --concurrency=1 --loglevel=info
+```
+
+Use `--concurrency=1` to limit resource usage on your dev machine.
+
+### Verifying the Setup
+
+1. **Beat** should log `Scheduler: Sending due task reports.scheduler (reports.scheduler)` once per minute
+2. **Worker** should log `Scheduling alert <name> eta: <timestamp>` for each active report
+3. Create a test report in **Settings > Alerts & Reports** with a `* * * * *` cron schedule
+4. Check **http://localhost:8025** (MailHog web UI) for the email within 1-2 minutes
+
+### Troubleshooting
+
+| Problem | Solution |
+|---|---|
+| Beat shows no output | Ensure `beat_schedule` is defined in your `CeleryConfig` and `--loglevel=info` is set |
+| "Report Schedule is still working, refusing to re-compute" | Previous executions are stuck. Reset with: `UPDATE report_schedule SET last_state = 'Not triggered' WHERE id = <id>;` |
+| Task backlog overwhelming the worker | Flush Redis: `redis-cli FLUSHDB`, then restart Beat and Worker |
+| Screenshot timeout | Ensure your frontend dev server is running and `WEBDRIVER_BASEURL` matches its URL |
+
 ---
 
 *This documentation is under active development. Check back soon for updates!*