Foreign-mint swap path returns 500 instead of handling melt fee shortfall #408
Description
Bug Description
When attempting to topup using a foreign-mint token (e.g., Mountainlake), the app attempts to melt the token into the primary mint. If the melt quote requires more sats than the token amount due to fees, the operation fails with a 500 Internal Server Error instead of returning a proper user-facing error.
Root Cause
The fee-handling bug in the foreign-mint swap path: the app tries to melt a token but the melt quote requires more sats than provided (including fees), causing an unhandled exception that surfaces as 500.
Reproduction
Case 1: 404 sat token
- Request ID: ef2c0cc1-0484-4165-a3d7-4298348a9b60
- Log location: logs/app_2026-03-14.log:53634
- Token: 404 sat from https://mint.mountainlake.io
- Error: Provided: 404, needed: 412 (Code: 11000)
- Result: 500 Internal Server Error
Case 2: 4200 sat token
- Request ID: c8bb32e1-0486-4b5b-b5cd-621b57735274
- Log location: logs/app_2026-03-14.log:55015
- Token: 4200 sat from https://mint.mountainlake.io
- Error: Provided: 4200, needed: 4284 (Code: 11000)
- Result: 500 Internal Server Error
Expected Behavior
The app should detect the insufficient melt inputs and return a user-friendly error message explaining that the token value is insufficient to cover melt fees, rather than returning a 500 Internal Server Error.
Additional Notes
- Both tokens are now SPENT at the mint (checked just now)
- The 404 sat request failed because the melt needed 412 sats
- The 4200 sat request failed because the melt needed 4284 sats
- This confirms the bug is reproducible and not token-specific