Add multi-tenant cloud mode: self-serve purchase, credit metering, core-decoupling

Introduces RECAP_MODE=multi alongside single-mode self-host:
- Tenant auth + accounts (magic-link via System SMTP), per-tenant credit pool,
  anonymous trial minting with per-IP/-64 caps
- Self-serve Pro/Max purchase: inline Lightning (BTCPay) + card (Zaprite),
  prepaid 30-day periods, expiry-reminder emails
- Core-decoupling: relay owns cloud tier/expiry keyed by Recaps user-id
- SQLite (better-sqlite3) schema for multi-mode; filesystem unchanged for single
- StartOS actions/versions through 0.2.155

startos/versions/v0.2.51.ts

@@ -0,0 +1,12 @@
+import { VersionInfo } from '@start9labs/start-sdk'
+
+export const v_0_2_51 = VersionInfo.of({
+  version: '0.2.51:0',
+  releaseNotes: {
+    en_US: 'Highlighted tier card now renders with a brighter purple border, an upward shift, and a stronger glow — much harder to miss than the previous subtle treatment. Policies cache shortened from 60s to 30s so operator edits in Keysat admin show up in the buy modal within ~30 seconds. (Note: to actually see the highlight, mark the policy as "Most popular" / Highlighted in Keysat admin first.)',
+  },
+  migrations: {
+    up: async ({ effects }) => {},
+    down: async ({ effects }) => {},
+  },
+})