FossFLOW: store diagrams on the host

[jackspiering]

Description

FossFLOW saves diagrams on the server in /data/diagrams (ENABLE_SERVER_STORAGE=true is the default of the image). The stack had no volume, so every saved diagram was lost when the container was recreated, for example after an image update.

• compose.yaml: add the volume ./${SERVICE}-data/diagrams:/data/diagrams.
• README.md (service): list the data folder, remove the data-loss warning, and add an "Upgrading" section that explains how to copy existing diagrams out of the container first.

Related Issues

• None.

Verification

Tested in a scratch copy with stnsmith/fossflow:latest. A stand-in container replaced Tailscale, because the test only concerns the volume. Diagrams were saved and listed through the API on port 80 (POST and GET /api/diagrams), the same path that the web interface uses.

Step	main	This branch
Save a diagram, then recreate the container	List is empty	Diagram is still there
docker compose down, then up -d	List is empty	Diagram is still there
New installation: save a diagram, recreate the container	List is empty	Diagram is still there, file is in ./fossflow-data/diagrams

Upgrade path from the README, starting on main with one saved diagram:

1. mkdir -p fossflow-data/diagrams
2. docker cp app-fossflow:/data/diagrams/. ./fossflow-data/diagrams/
3. docker compose up -d with the file from this branch

Result: the diagram from the old container is listed, next to a new one that was saved afterwards.

• docker compose config --quiet in services/fossflow: passed.
• rumdl check --config .markdownlint.yml services/fossflow/README.md: passed.
• git diff --check: passed.

Not tested: access through the Tailnet, which this change does not touch.

Checklist

• I have performed a self-review of my code and followed the templates structure.
• I have added verification that the stack works as expected.
• I have updated necessary documentation (e.g. frontpage README.md ).
• I have selected the correct label(s) for this PR.

Additional Context

• This pull request is stacked on All services: standardize the service READMEs #363 and targets its branch, because it edits the new README. Merge All services: standardize the service READMEs #363 first.
• The original repository stan-smith/FossFLOW no longer exists on GitHub. The image stnsmith/fossflow is still on Docker Hub, and Abrar74774/FossFLOW continues the project. Whether the stack should move to another image is a separate question.