Guard mock billing upgrades in production

[AlexKitipov]

Motivation

• Prevent demo/mock premium upgrades from being accidentally exposed in production by gating the mock endpoint behind an explicit configuration flag.
• Provide placeholders for a future Stripe checkout integration so production billing can be wired in safely.

Description

• Add ENABLE_MOCK_BILLING and Stripe placeholder settings (STRIPE_API_KEY, STRIPE_WEBHOOK_SECRET, STRIPE_PRICE_ID, STRIPE_SUCCESS_URL, STRIPE_CANCEL_URL) to app/config.py.
• Gate the mock upgrade flow in app/api/v1/users.py with a helper _mock_billing_enabled() and return a clear 403 MOCK_BILLING_DISABLED AppException when ENVIRONMENT=production and ENABLE_MOCK_BILLING=false.
• Update README.md to document the production guard and Stripe placeholders.
• Add tests tests/test_users_upgrade.py that assert the mock upgrade is blocked in production by default and allowed when ENABLE_MOCK_BILLING=true for controlled demos.

Testing

• Ran the new unit tests with pytest -q tests/test_users_upgrade.py, which passed.
• Ran the full test suite with pytest -q, which completed successfully (all tests passed, with only deprecation warnings reported).

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