feat: add UMS CLI for binary packaging and system initialization

- Add Cobra-based CLI with ums init, ums serve, ums version commands
- ums init supports interactive prompts and non-interactive flags
- Generates secure JWT secrets and config.yaml automatically
- Extract server.Serve() function for reuse
- Add cross-platform build targets to Makefile
- Update README with CLI installation and usage instructions

New files:
- cmd/ums/main.go - CLI entry point
- cmd/ums/cmd/root.go - Root command
- cmd/ums/cmd/init.go - Interactive/non-interactive init
- cmd/ums/cmd/serve.go - Server command
- cmd/ums/cmd/version.go - Version command
- internal/server/server.go - Extracted Serve function

go.sum

@@ -59,6 +59,7 @@ github.com/client9/misspell v0.3.4/go.mod h1:qj6jICC3Q7zFZvVWo7KLAzC3yx5G7kyvSDk
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@@ -171,6 +172,8 @@ github.com/icholy/digest v1.1.0 h1:HfGg9Irj7i+IX1o1QAmPfIBNu/Q5A5Tu3n/MED9k9H4=
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@@ -229,6 +232,7 @@ github.com/prometheus/procfs v0.13.0 h1:GqzLlQyfsPbaEHaQkO7tbDlriv/4o5Hudv6OXHGK
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@@ -244,6 +248,7 @@ github.com/richardlehane/msoleps v1.0.4 h1:WuESlvhX3gH2IHcd8UqyCuFY5yiq/GR/yqaSM
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@@ -257,8 +262,12 @@ github.com/spf13/afero v1.11.0 h1:WJQKhtpdm3v2IzqG8VMqrr6Rf3UYpEF239Jy9wNepM8=
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@@ -423,6 +432,8 @@ golang.org/x/term v0.17.0/go.mod h1:lLRBjIVuehSbZlaOtGMbcMncT+aqLLLmKrsjNrUguwk=
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+golang.org/x/term v0.41.0 h1:QCgPso/Q3RTJx2Th4bDLqML4W6iJiaXFq2/ftQF13YU=
+golang.org/x/term v0.41.0/go.mod h1:3pfBgksrReYfZ5lvYM0kSO0LIkAl4Yl2bXOkKP7Ec2A=
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