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/v0.2.121.ts

@@ -0,0 +1,13 @@
+import { VersionInfo } from '@start9labs/start-sdk'
+
+export const v_0_2_121 = VersionInfo.of({
+  version: '0.2.121:0',
+  releaseNotes: {
+    en_US:
+      "Remove the 'N segments' counter from the results header, stats bar, video metadata line, and podcast metadata line. The raw transcript-segment count (often 600+) was a noisy implementation detail that didn't tell viewers anything useful — knowing a 90-minute video has 637 segments doesn't help anyone decide whether to read it. The header now reads simply 'N topics · M:SS total' across all variants (video, podcast, and the live-streaming header counter). Topic count and total duration are the actual viewer-facing numbers worth surfacing.",
+  },
+  migrations: {
+    up: async ({ effects }) => {},
+    down: async ({ effects }) => {},
+  },
+})