billing: Start billing cycle on the 1st each month

[ZJvandeWeg]

For billing insights and understand early in the month about where our revenue is going to end I've updated the billing cycle to start the 1st of each month. The default behaviour is to start the month the date + time they start their subscription.

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production

Labels

• Backport needed? -> add the backport label
• Includes a DB migration? -> add the area:migration label

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 79.25%. Comparing base (05a4c0d) to head (71d525f).
Report is 7 commits behind head on main.

❗ Current head 71d525f differs from pull request most recent head 4b614fe

Please upload reports for the commit 4b614fe to get more accurate results.

Files	Patch %	Lines
forge/ee/lib/billing/index.js	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3748      +/-   ##
==========================================
- Coverage   79.26%   79.25%   -0.01%     
==========================================
  Files         281      281              
  Lines       12723    12728       +5     
  Branches     2838     2838              
==========================================
+ Hits        10085    10088       +3     
- Misses       2638     2640       +2     

Flag	Coverage Δ
backend	79.25% <75.00%> (-0.01%)	⬇️

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[knolleary]

billing_cycle_anchor_config isn't a valid parameter for the stripe.checkout.sessions.create(..) function we use.

According to the docs, we have to use subscription_data.billing_cycle_anchor which lacks the convenice of billing_cucle_anchor_config as we'll have to calculate the timestamp of the next 1st of the month ourselves.

[ZJvandeWeg]

@knolleary I'm sorry; I don't quite now how to validate my changes. The only test I could think of was to spy on the map/hash constructed, which felt silly. But I don't have a great Stripe test setup. Do you know how to achieve this and validate my hacking?

[knolleary]

@ZJvandeWeg I'll take a look. I have stripe setup locally; what is harder to test is time-based events in the future where you want to check what happens at invoice time etc. That's something we have to do fairly manually.

[knolleary]

Initial test looks good. Here is what the checkout page shows:

Are we happy with that?

[ZJvandeWeg]

I like this a lot, thanks @knolleary !