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

server/audio.js

@@ -53,7 +53,18 @@ export async function splitAudioFile(inputPath, outputDir, chunkSeconds = 2700)
       "-acodec", "copy",
       chunkPath,
     ], { timeout: 120000 });
-    chunks.push({ path: chunkPath, startOffset: startSec, index: i });
+    chunks.push({
+      path: chunkPath,
+      startOffset: startSec,
+      // Actual seconds in THIS chunk (the last chunk is usually
+      // shorter than chunkSeconds). Carried downstream so the
+      // transcribe-stitching code can sanity-cap timestamps each
+      // chunk's model emits — some models hallucinate offsets
+      // way past the chunk's audio (observed: gemini-3.1-flash-lite
+      // emitting [10:12:44] on a 45-min chunk).
+      durationSec: segLen,
+      index: i,
+    });
     startSec += chunkSeconds;
     i++;
   }