[Draft] Bootstrap initial Stripe billing foundation

[BASIC-BIT]

Problem

VRDex has a locked billing direction (Stripe), but there is not yet a concrete first-pass Stripe setup in the repo.

Scope

• bootstrap the minimum Stripe foundation needed for the locked stack direction
• establish the initial integration shape for:
  • customer creation
  • subscription state handling direction
  • billing portal direction
  • webhook ingestion direction
• keep the first pass aligned with the documented rule that product behavior should read internal entitlement state rather than Stripe directly everywhere
• keep the setup friendly to later paid-tier work without overbuilding monetization too early

Non-goals

• final plan packaging
• full premium feature implementation
• polished billing UI
• complete entitlement model for every future paid feature
• production-ready tax/compliance work
• deep finance/reporting workflows

Acceptance criteria

• the repo has a concrete first-pass Stripe integration/bootstrap path
• the initial direction for customer creation is explicit
• the initial direction for webhook handling is explicit
• the initial direction for internal entitlement synchronization is explicit
• the setup is narrow enough to support early billing architecture without forcing premature product packaging decisions

Likely docs to update

• README.md
• docs/planning/engineering-strategy.md
• docs/README.md
• any billing/bootstrap doc created during implementation

Soft dependencies

• #54 Bootstrap initial Convex backend, if internal entitlement state is established there
• should stay aligned with the billing architecture guidance in docs/planning/engineering-strategy.md

Soft dependents

• paid-tier implementation issues
• entitlement/state issues
• billing portal implementation
• webhook auditability work
• premium customization or analytics features