Refactor backupCreate for testability

[ivanauth]

Summary

• Extract backupCreateImpl with injectable client and ProgressTracker interface for testability
• Harden retry logic (error checks, max 10 retries)
• Remove legacy Serverless backup path
• Add --prefix-filter and --rewrite-legacy flags

Resolves #569

[codecov-commenter]

Codecov Report

❌ Patch coverage is 51.01010% with 97 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.82%. Comparing base (ec981d0) to head (7a02c9f).

Files with missing lines	Patch %	Lines
internal/cmd/backup.go	51.01%	73 Missing and 24 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #604      +/-   ##
==========================================
+ Coverage   42.48%   42.82%   +0.34%     
==========================================
  Files          38       38              
  Lines        5030     5083      +53     
==========================================
+ Hits         2137     2177      +40     
+ Misses       2638     2634       -4     
- Partials      255      272      +17     

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