dev-stack: enable LNBITS_DEMO_MODE for transport-driver pay flow

The nostr transport's --flow subscribe driver flow creates two fresh
accounts via the transport's auto-account-creation path and needs the
payer one to have funds. LNBITS_DEMO_MODE auto-credits 1M sats to the
default wallet on account creation (gated to FakeWallet + this opt-in,
see lnbits/core/services/users.py:_demo_mode_enabled).

The previous round's nostr-transport env wiring already lives in this
file; this adds the demo flag so the new --flow subscribe demo runs
out of the box.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docker-compose.dev.yml

@@ -36,6 +36,11 @@ services:
       # will successfully connect (the reconnect loop retries every 5s).
       NOSTR_TRANSPORT_ENABLED: "true"
       NOSTR_TRANSPORT_RELAYS: '["ws://localhost:5001/nostrrelay/test"]'
+      # Auto-credit 1M sats to freshly-created accounts. Required by the
+      # transport-driver `--flow subscribe` end-to-end test which pays a
+      # FakeWallet invoice between two newly created wallets. Gated to
+      # FakeWallet + this explicit opt-in (see lnbits/core/services/users.py).
+      LNBITS_DEMO_MODE: "true"
     ports:
       - 5001:5001
     volumes: