feat(restore): add retry with exponential backoff for restore operations

[benbjohnson]

Summary

• Add RetryReplicaClient wrapper that retries LTXFiles() and OpenLTXFile() on transient errors
• Does NOT retry on os.ErrNotExist (legitimate "not found" response) or context errors
• Uses configurable exponential backoff (delay doubles each retry, capped at MaxDelay)
• Integrates with litestream restore command via new flags

New Flags

Flag	Default	Description
-retry-delay	1s	Initial delay between retries
-retry-max-delay	30s	Maximum delay (exponential backoff cap)
-retry-max-retries	5	Max retry attempts (0 to disable)

Usage

# Restore with default retry settings (enabled by default)
litestream restore -o /tmp/db s3://bucket/db

# Disable retries
litestream restore -retry-max-retries 0 -o /tmp/db s3://bucket/db

# Custom retry config
litestream restore -retry-delay 2s -retry-max-delay 60s -retry-max-retries 10 -o /tmp/db s3://bucket/db

Test plan

• Unit tests for RetryReplicaClient (14 test cases)
• Manual test with S3 backend
• Verify no retry on missing files

Closes #1041

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