feat: add comprehensive logging to payment cron for debugging

Every 10 minutes, the cron now logs:
- When it triggers and completes
- Satoshi rate fetched (or failure)
- Number of eligible shareholders found
- If 0 found: warns about shareholders with revenue but mismatched criteria
- Per-shareholder: revenue, filmmaker ID, lightning address
- Payment preparation: amount in sats, destination address
- Duplicate detection
- Payment send result (completed or failed with error)
- Revenue deduction confirmation

Co-authored-by: Cursor <cursoragent@cursor.com>

docker-compose.yml

@@ -47,7 +47,7 @@ services:
       context: ./backend
       dockerfile: Dockerfile
       args:
-        CACHEBUST: "12"
+        CACHEBUST: "13"
     restart: unless-stopped
     environment:
       # ── Core ─────────────────────────────────────────────