fix(cash-in): return bech32 LNURL, not the raw URL

`Lnurl.__str__` is the underlying URL, so `str(lnurl)` returned
`http://<baseurl>/withdraw/...` instead of the bech32 `LNURL1…` — wallets
need the encoded LNURL-withdraw (lud01). Use `str(lnurl.bech32)` and add
`lnurl_url` (the raw URL) alongside, mirroring withdraw's _populate_lnurl
field convention. (Note: the encoded URL still derives from LNBITS_BASEURL —
that must be an externally reachable https URL for a real wallet to claim.)

cashin_transport.py

@@ -107,7 +107,10 @@ async def handle_create_withdraw(auth, request) -> dict:
     )
     return {
         "link_id": link.id,
-        "lnurl": str(lnurl),
+        # `Lnurl.__str__` is the raw URL — wallets need the bech32 LNURL1…
+        # (lud01). Mirror withdraw's `_populate_lnurl` field convention.
+        "lnurl": str(lnurl.bech32),
+        "lnurl_url": str(lnurl.url),
         "net_sats": net_sats,
         "principal_sats": principal_sats,
         "fee_sats": fee_sats,