This PR adds error tracking and retry logic for channel monitor retrieval during React Native to native Android migration, preventing silent channel loss.

Description

During migration from the RN app, channel monitors are fetched from the remote backup. Previously, if any monitor retrieval failed (network timeout, decryption error, etc.), it was silently dropped via .mapNotNull(), potentially causing permanent channel loss without any indication to the user or in logs.

This PR:

1. Tracks which specific channel monitors fail to retrieve
2. Logs failed channel IDs for debugging
3. Adds a retry mechanism (3 attempts with linear backoff: 1s, 2s, 3s)
4. Warns when monitor count doesn't match expected

Preview

N/A - Backend migration logic, no UI changes

QA Notes

1. Migration with all monitors available

1. Perform an RN to native migration with a wallet that has channels
2. Verify migration completes successfully
3. Verify channels are available after migration

2. Log verification (requires simulated failure)

1. If a monitor fails transiently: should see "Retrying channel monitor retrieval..." debug logs
2. If a monitor fails permanently: should see "Failed to retrieve X/Y channel monitors after retries" error log
3. Should see "Channel monitor count mismatch" warning if any monitors are lost

3. Regression - Normal operation

1. Verify normal wallet operation is unaffected
2. Verify lightning payments work as expected