Backend: migrate ENABLE_DONATIONS to donations_enabled feature flag

[aapeliv]

Migrates the ENABLE_DONATIONS env toggle to a donations_enabled feature flag (GrowthBook), so donations can be turned on/off remotely without a deploy. This is the first of Project 1 — moving ENABLE_* functionality toggles onto the feature flag system.

Policy this establishes for ENABLE_* → flag migrations:

• The runtime gate becomes a per-request flag: context.get_boolean_value("donations_enabled", default=False) (fails closed when experimentation is off/unconfigured).
• Paired credentials (STRIPE_*) stay in env. Since the flag can be flipped on remotely at any time, the boot-time guarantee can no longer be gated on an enable-bit, so prod now requires the Stripe creds unconditionally (moved into the if not DEV: block) and screams loudly at boot if any are missing.
• No runtime degrade-gracefully guard: prod is guaranteed configured, so a flip always works.

Ops: create donations_enabled in GrowthBook (default = prod's current donations state) before/at deploy. Until configured, the in-code default (False) applies.

Testing

• make format clean; make mypy reports only the 2 pre-existing unrelated errors.
• test_donations.py 10/10 — dropped the now-unnecessary ENABLE_DONATIONS=True patches (the flag defaults on in tests via EXPERIMENTATION_PASS_ALL_GATES), and added test_donations_disabled, which uses the shared feature_flags fixture to force the gate off and asserts the UNAVAILABLE / "Donations are currently disabled." abort.
• Adjacent suites (experimentation/account/public) 71/71.

Backend checklist

• Added tests for any new code or added a regression test if fixing a bug
• Run the backend locally and it works
• Added migrations if there are any database changes, rebased onto develop if necessary for linear migration history (n/a — no DB changes)

For maintainers

• Maintainers can push commits to my branch
• Maintainers can merge this PR for me

This PR was created with the Couchers PR skill.

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Project	Deployment	Actions	Updated (UTC)
couchers	Ready	Preview	May 23, 2026 10:08pm

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📝 Release Notes

This PR does not need to be included in release notes.

Reason: This change is primarily an internal backend/configuration migration from an environment variable to a feature flag. While it improves operational flexibility for turning donations on or off without a deploy, it does not add or noticeably change functionality for end users, and it is not a major enough technical infrastructure change to surface in user-facing release notes.

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: exclude
Reasoning: This change is primarily an internal backend/configuration migration from an environment variable to a feature flag. While it improves operational flexibility for turning donations on or off without a deploy, it does not add or noticeably change functionality for end users, and it is not a major enough technical infrastructure change to surface in user-facing release notes.