Enterprise: org-funded checkout, wallet, and member dashboard (wallet flag)

[teetangh]

Context

The MVP ships read-only org context: GET /api/me/organization, GET /api/me/program-assignments (entitlement math for LICENSED_SEAT/CREDIT_POOL), a My Organization screen, an org entry on Profile, and OrgPlanVisibility filtering in explore (ORG_ONLY plans visible to members). All enterprise WRITES are out of scope and FeatureFlags.wallet = false.

Scope (depends on payments #113)

1. Org-funded checkout: FundingSource resolution (PERSONAL|LICENSE|WALLET|INVOICE), PaymentLeg stacking, BookingUtilization creation, ProgramAssignment cap checks with overage behavior (BLOCK|CHARGE_MEMBER|CHARGE_ORG) — web's checkout uses Redis distributed locks + SERIALIZABLE transactions; mobile must NOT reimplement this independently (see Critical: Race condition risks when mobile and web backends run simultaneously #19, refactor(payments): Swiggy-style architecture - Web-only webhook processing with mobile read-only access #15 — likely route through one checkout owner)
2. Member-facing utilization history (BookingUtilization list per assignment)
3. Org-scoped maintenance windows + announcements
4. Org admin surfaces (members, invoices, contracts) — explicitly deferred, likely web-only forever
5. Reference: familiarise_web docs/enterprise/ (00-foundations, 30-programs-and-lifecycle)

Related: #113, #15, #19, #18