Add subscription info to invite emails & endpoint for sending "trial ending soon" emails

[emma-sg]

Closes #2685
Closes #2798

Changes

• Adds subscription info (specifically days until trial ends) to invite emails
• Adds new endpoint for sending a "trial ending soon" email to all admins based on subscription id

Testing

Tested in dev alongside https://github.com/webrecorder/cashew/pull/57! All seems to work.

Tested locally by doing the following:

1. Rebuild the backend and emails images with scripts/build-backend.sh and scripts/build-emails.sh
2. Set a value for BTRIX_SUBS_APP_API_KEY in the btrix-subs-app-secret secret by doing the following:
   1. Choose a secret and write it down — I just used uuidgen to generate one
   2. Base64-encode the secret, I used cat '<my secret>' | base64
   3. Run kubectl edit secrets btrix-subs-app-secret and paste your encoded secret in the BTRIX_SUBS_APP_API_KEY field, and save and exit
3. Redeploy your local Browsertrix with helm upgrade --install -f ./chart/values.yaml -f ./chart/local.yaml btrix ./chart/ && kubectl wait --for=condition=ready pod --all --timeout=300s
4. Manually modify the subscription field on the org document in MongoDB
   I used the following data in MongoDB Compass connected to my local Browsertrix instance:

   "subscription": {
     "subId": "stripe:sub_abc123",
     "planId": "starter",
     "status": "trialing",
     "futureCancelDate": {
       "$date": "2025-08-14T21:14:29.000Z"
     },
     "readOnlyOnCancel": false
   },

   Pick some date a bit into the future for the futureCancelDate.
5. Add an admin user or two to an org, and follow the sign-up process
   • Verify that the "Manage Your Billing and Plan" section on the sign-up emails mentions when the trial ends
6. Set up smtp4dev or something similar to receive emails sent from Browsertrix (see Testing Email Sending from Browsertrix)
7. Run the following to get Browsertrix to send trial ending soon emails to all the admins of your org:

   curl -X 'POST' \
     'http://browsertrix-cloud-backend.default.svc.cluster.local:8000/api/subscriptions/trial-end-reminder' \
     -H 'Content-Type: application/json' \
     -d '{
     "subId": "stripe:sub_abc123",
     "behavior_on_trial_end": "cancel"
   }' -H 'Authorization: bearer <your secret from step 2.i>'

   Try with behavior_on_trial_end set to both "cancel" and "continue".
   • Verify that the dates & durations are accurate
   • Verify that the links to the org all work
   • Verify that all admins receive emails

Follow-ups

Once this is deployed, we should turn off trial end reminder emails from Stripe.

[tw4l]

Tested on dev, working well. Left a few small suggestions.

[ikreymer]

Small nitpick: I think should just use SuccessResponse(success=True) rather than adding a new API response type?

[emma-sg]

Oh sure, yeah