feat: Migrate AWS backends from SDK v1 to v2

[abtreece]

Summary

• Migrate SSM, ACM, and DynamoDB backends from AWS SDK v1 to v2
• Add context.Context parameter to StoreClient interface methods
• Update all backend implementations for new interface signature
• Remove AWS SDK v1 dependency

Closes #354

Breaking Change

The StoreClient interface now requires context.Context as the first parameter:

type StoreClient interface {
    GetValues(ctx context.Context, keys []string) (map[string]string, error)
    WatchPrefix(ctx context.Context, prefix string, keys []string, waitIndex uint64, stopChan chan bool) (uint64, error)
}

Test plan

• All unit tests pass
• Integration tests pass (CI will verify)
• Test SSM backend against LocalStack
• Test ACM backend against LocalStack
• Test DynamoDB backend against LocalStack

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 73 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.05%. Comparing base (c91912d) to head (4bae427).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/backends/acm/client.go	30.55%	25 Missing ⚠️
pkg/backends/ssm/client.go	46.66%	24 Missing ⚠️
pkg/backends/dynamodb/client.go	54.16%	20 Missing and 2 partials ⚠️
pkg/backends/vault/client.go	50.00%	1 Missing ⚠️
pkg/template/processor.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #355      +/-   ##
==========================================
- Coverage   48.36%   48.05%   -0.31%     
==========================================
  Files          20       20              
  Lines        1863     1881      +18     
==========================================
+ Hits          901      904       +3     
- Misses        881      895      +14     
- Partials       81       82       +1     

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