Fix billing downgrade recharge and suspension race

[niemyjski]

Summary

• Prorate and finalize paid-to-free cancellations so unused yearly time becomes customer credit before a later re-upgrade.
• Serialize plan changes and subscription webhooks per organization, and ignore stale or free-plan cancellation events.
• Centralize system IDs while preserving existing public compatibility aliases.
• Add signed local endpoint coverage for event ordering, stale/current subscriptions, concurrent changes, and reactivation.

Root cause

Paid-to-free changes immediately canceled the subscription without proration. A later upgrade therefore created a new full-price subscription. Asynchronous deletion webhooks could also overwrite the free or newly paid state because billing updates were neither serialized nor correlated to the current subscription.

Verification

• dotnet build Exceptionless.slnx -m:1 — 0 warnings, 0 errors
• BillingLifecycleEndpointTests — 11 passed
• OrganizationEndpointTests — 84 passed
• Full backend suite — 2,510 passed, 0 failed, 2 intentional skips
• Merged-head localhost Aspire dogfood — API and Svelte health returned HTTP 200
• Local-only Stripe boundary — no Stripe API traffic, Stripe.js, real/test keys, or production URLs
• Thermonuclear merged-head review — no remaining race, compatibility, merge, or code-quality blocker

Breaking changes

None.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
Exceptionless.Core	73%	65%	9509	➖
Exceptionless.AppHost	38%	40%	139	❌
Exceptionless.Insulation	23%	23%	205	❌
Exceptionless.Web	84%	67%	6900	✔
Summary	77% (22593 / 29475)	65% (10366 / 15836)	16753	✔