Add env-controlled strict Stripe checkout guard

[AJaySi]

Motivation

• Make Stripe integration opt-in-strict for demos or production so missing Stripe keys fail loudly when required.
• Preserve the current fallback behavior in environments that do not opt into strict mode.

Description

• Add _is_truthy_env helper and wire REQUIRE_STRIPE_CHECKOUT into StripeService to enable strict backend mode.
• When REQUIRE_STRIPE_CHECKOUT=true and STRIPE_SECRET_KEY is missing, StripeService now raises a clear HTTPException(500) instead of allowing silent fallback.
• Add frontend handling via REACT_APP_REQUIRE_STRIPE_CHECKOUT and REACT_APP_STRIPE_PUBLISHABLE_KEY checks in PricingPage to force the Stripe checkout path and throw errors when the publishable key is missing or checkout URL is not returned.
• Surface strict-mode error messages in the UI by propagating the actual error string to the page error banner.

Testing

• Ran python -m compileall backend/services/subscription/stripe_service.py which succeeded.

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Codex Task