Harvey/stripe paid signup

[heywander]

Adds optional, disabled-by-default Stripe paid signup support to jfa-go.

This is the cleaned-up upstream-facing version of the work, split into 8 logical commits for
review instead of one large branch. It includes configurable payment plans, Stripe checkout/
webhook handling, purchased invite emails, user subscription status, admin payment management, and
reconciliation tasks. Normal invite behavior remains unchanged unless Stripe is enabled.

[netlify]

👷 Deploy request for jfa-go pending review.

Visit the deploys page to approve it

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🔨 Latest commit	70a0e3f

[heywander]

For context, the earlier PRs were messy because I was still actively building and testing the demo:

#460
#462

This PR is the cleaned-up upstream-facing version, split into logical commits and stripped of my
fork/build/release setup.

If anyone wants to try the working fork before this is reviewed or merged (if at all), it’s here:

https://github.com/heywander/jfa-go-pay

@Action3062