docs(dashboard): add local verification rig for Excel export

Adds a compose overlay (MinIO as S3 stand-in + Mailpit as SMTP catcher,
with auto bucket creation) and a runbook so reviewers can verify the
async Excel export end-to-end locally: UI button -> Celery worker ->
S3 upload -> emailed pre-signed link -> downloadable .xlsx.

Runs under a distinct compose project name with conflicting host port
bindings reset, so it coexists with other local superset stacks.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

VERIFY_EXCEL_EXPORT.md

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+<!--
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License.  You may obtain a copy of the License at
+
+  http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
+software distributed under the License is distributed on an
+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
+
+# Verifying the dashboard "Export Data to Excel" feature locally
+
+Everything runs in Docker under the compose project `amman-excel`, fully
+isolated from your other superset stacks (distinct project name, no host-port
+collisions: redis/db host bindings are dropped by the overlay).
+
+## One-time setup
+
+1. Create `docker/pythonpath_dev/superset_config_docker.py` (gitignored) with
+   the S3/SMTP wiring for the overlay services:
+
+   ```python
+   EXCEL_EXPORT_S3_BUCKET = "dashboard-exports"
+   EXCEL_EXPORT_S3_CLIENT_KWARGS = {
+       "endpoint_url": "http://minio:9100",
+       "aws_access_key_id": "minioadmin",
+       "aws_secret_access_key": "minioadmin",
+       "region_name": "us-east-1",
+   }
+
+   # Mailpit: plain SMTP, no TLS, no auth (defaults would try STARTTLS + login).
+   SMTP_HOST = "mailpit"
+   SMTP_PORT = 1025
+   SMTP_STARTTLS = False
+   SMTP_SSL = False
+   SMTP_USER = None
+   SMTP_PASSWORD = None
+   SMTP_MAIL_FROM = "superset@example.com"
+   ```
+
+2. The pre-signed download link in the email points at `http://minio:9100/...`.
+   For your **browser** to resolve `minio`, add one line to /etc/hosts:
+
+   ```bash
+   sudo sh -c 'echo "127.0.0.1 minio" >> /etc/hosts'
+   ```
+
+   (Skip this if you only verify via curl inside the network.)
+
+## Start / stop
+
+```bash
+# start (first boot: installs deps + loads example dashboards, ~5-10 min)
+docker compose -p amman-excel \
+  -f docker-compose.yml -f docker-compose-excel-verify.yml \
+  up -d superset superset-worker superset-init minio createbucket mailpit
+
+# watch until init finishes
+docker compose -p amman-excel logs -f superset-init | tail -5
+
+# stop (keeps data)     / tear down completely (incl. volumes)
+docker compose -p amman-excel down
+docker compose -p amman-excel down -v
+```
+
+## The verification walk
+
+| Where | What |
+|---|---|
+| http://localhost:8088 | Superset UI — login `admin` / `admin` |
+| http://localhost:8025 | Mailpit — the "inbox" that catches the export email |
+| http://localhost:9101 | MinIO console (`minioadmin`/`minioadmin`) — see the uploaded .xlsx |
+
+1. Open any example dashboard (e.g. "Sales Dashboard").
+2. Header `⋯` menu → **Export Data to Excel** → confirm the toast.
+3. Watch the worker: `docker compose -p amman-excel logs -f superset-worker`
+   — expect `Task export_dashboard_excel[...] succeeded`.
+4. Open Mailpit (localhost:8025) → email "[Report] Your dashboard export is
+   ready: …" → click the download button → the .xlsx downloads.
+5. Open the workbook: one sheet per chart, named `<chart id> - <chart name>`;
+   charts without a saved query context are listed in the email as omitted.
+
+## Negative paths worth clicking
+
+- Comment out `EXCEL_EXPORT_S3_BUCKET` in
+  `docker/pythonpath_dev/superset_config_docker.py`, restart the `superset`
+  service, and hit the endpoint → clean **501** "not configured".
+- A dashboard with no charts → **400** "Dashboard has no charts to export."
+- Tamper any character of the pre-signed URL's signature → MinIO rejects it.
+
+## Files that make this work
+
+- `docker-compose-excel-verify.yml` — compose overlay: MinIO + Mailpit +
+  bucket bootstrap, resets conflicting redis/db host ports.
+- `docker/pythonpath_dev/superset_config_docker.py` — gitignored local
+  override (create it from the snippet above): S3 client kwargs pointing at
+  MinIO, SMTP pointing at Mailpit.
+- `docker/pythonpath_dev/superset_config.py` — adds
+  `superset.tasks.export_dashboard_excel` to the docker dev CeleryConfig
+  imports (without it, docker-compose exports silently no-op).

docker-compose-excel-verify.yml

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+#
+# Licensed to the Apache Software Foundation (ASF) under one or more
+# contributor license agreements.  See the NOTICE file distributed with
+# this work for additional information regarding copyright ownership.
+# The ASF licenses this file to You under the Apache License, Version 2.0
+# (the "License"); you may not use this file except in compliance with
+# the License.  You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# Local-verification overlay for the dashboard "Export Data to Excel" feature.
+# Adds MinIO (S3 stand-in) + Mailpit (SMTP catcher) and un-binds the redis/db
+# host ports that collide with other containers on this machine.
+#
+# Usage (distinct project name so it can't touch your other superset stacks):
+#   docker compose -p amman-excel \
+#     -f docker-compose.yml -f docker-compose-excel-verify.yml \
+#     up -d superset superset-worker superset-init minio createbucket mailpit
+services:
+  minio:
+    image: minio/minio
+    # Listen on 9100 (not the default 9000) so the pre-signed URL port works
+    # identically inside the network and from your browser via /etc/hosts.
+    command: server /data --address ":9100" --console-address ":9101"
+    environment:
+      MINIO_ROOT_USER: minioadmin
+      MINIO_ROOT_PASSWORD: minioadmin
+    ports:
+      - "127.0.0.1:9100:9100"
+      - "127.0.0.1:9101:9101"
+    volumes:
+      - minio_data:/data
+
+  createbucket:
+    image: minio/mc
+    depends_on:
+      - minio
+    entrypoint: >
+      /bin/sh -c "
+      sleep 3;
+      mc alias set local http://minio:9100 minioadmin minioadmin &&
+      mc mb --ignore-existing local/dashboard-exports;
+      exit 0"
+
+  mailpit:
+    image: axllent/mailpit
+    ports:
+      - "127.0.0.1:8025:8025"
+
+  # Drop host port bindings that collide with other local containers
+  # (another workspace's redis on 6379, a standalone postgres on 5432).
+  redis:
+    ports: !reset []
+  db:
+    ports: !reset []
+
+volumes:
+  minio_data: