Alby Hub v1.22.2 fails to start with Cashu/Minibits backend: error saving keyset - bucket name required

## Bug Description
Alby Hub v1.22.2 (running on Start9) fails to start when using Minibits (Cashu) as the LN backend. The error "error saving keyset: bucket name required" appears consistently on every start attempt. Two incoming payments were made but never issued to the wallet, and the Hub cannot start to claim them.

## Steps to Reproduce
1. Configure Alby Hub on Start9 with Minibits (Cashu) as the LN backend at https://mint.minibits.cash/Bitcoin
2. Attempt to start Alby Hub → immediately fails with keyset error
3. Hub cannot proceed past the password screen

## Logs (from unsuccessful start attempts)
```
2026-08-27T01:05:40-06:00 {"level":"info","msg":"Launching LN Backend: CASHU","time":"2026-08-27T07:05:40Z"}
2026-08-27T01:05:41-06:00 {"level":"info","mintUrl":"https://mint.minibits.cash/Bitcoin","msg":"Setting up cashu wallet","time":"2026-08-27T07:05:41Z"}
2026-08-27T01:05:42-06:00 {"error":"error saving keyset: bucket name required","level":"error","msg":"Failed to load cashu wallet","time":"2026-08-27T07:05:42Z"}
2026-08-27T01:05:42-06:00 {"error":"error saving keyset: bucket name required","level":"error","msg":"Failed to launch LN backend","time":"2026-08-27T07:05:42Z"}
2026-08-27T01:05:42-06:00 {"level":"error","msg":"Failed to launch LN backend: error saving keyset: bucket name required","time":"2026-08-27T07:05:42Z"}
2026-08-27T01:05:42-06:00 {"error":"error saving keyset: bucket name required","level":"error","msg":"Failed to start node","time":"2026-08-27T07:05:42Z"}
```

## Additional Context
- **Hardware**: Start9 Embassy
- **OS**: Start9 OS
- **Alby Hub version**: 1.22.2
- **LN Backend**: Cashu (Minibits) at https://mint.minibits.cash/Bitcoin
- **Minibits response**: "bucket name" is not a Cashu concept — likely an Alby Hub internal issue when initiating the wallet connection
- **Minibits keyset rotation**: Keys rotated to new format (August 2026 migration) — possible compatibility issue with wallet implementations
- **Two stuck payments** (confirmed by Minibits as "paid but not yet issued"):
- `24ebfbe07c293777ac3452da7ffe28132f9f80c213f41129b5f6a14d982fef64`
- `f5ce59e0bf87e0d88c0886f4e25118957bf423a821290abe5bf8fc0990bfe336`
- **Current state**: Cannot resolve — Hub still cannot start. Minibits says wallet should fire mint request to receive the ecash, but Hub won't start to do it.

## Expected Behavior
Alby Hub should start successfully with the Cashu/Minibits backend and be able to mint received payments.

## Environment
- Hardware: Start9 Embassy
- OS: Start9 OS
- Alby Hub version: 1.22.2
- LN Backend: Cashu (Minibits) at https://mint.minibits.cash/Bitcoin

Start by tracing Cashu/Minibits wallet initialization from the LN backend launch path and inspect where the reported “error saving keyset: bucket name required” is produced. Reproduce with the documented mint URL and verify that Alby Hub starts and can mint the two received payments; the issue provides no named source file or test.

Written by the indexing model from the issue text.