fix(cmd): address security, bugs, and maintainability in plikd CLI

[camathieu]

What

Comprehensive review and fix of the plikd server CLI (server/cmd/), addressing security issues, bugs, maintainability, and documentation gaps.

Why

A critical review of the CLI code surface uncovered several issues ranging from a panic on plikd import with no arguments, to a dangerous silent fallthrough to mass-delete in file delete, to credential leakage in log output.

Changes

Security

• Remove DB connection strings from logs — export.go, import.go no longer print ConnectionString (may contain credentials)
• Require explicit flag for file delete — file delete now requires --file, --upload, or --all instead of silently falling through to mass-delete when no flag is provided
• Fix user delete ordering — deleteUser now soft-deletes uploads → cleans data backend → deletes user metadata, preventing orphaned blobs

Bugs

• Fix panic in import — plikd import with no arguments would panic due to missing os.Exit(1) after error message
• Fix os.Exit in sync.Once — Init functions now store errors and check after Do(), avoiding fragile os.Exit inside closures
• Fix copy-paste errors in token.go — Three error messages incorrectly referenced "users" instead of "tokens"
• Refactor signal handling — Main goroutine now blocks on signal channel directly, enabling proper cleanup and error checking on Shutdown()

Maintainability

• Extract parseMaxSize/parseMaxTTL helpers — Removes ~50 lines of duplicated parsing logic between createUser and updateUser
• Use cmd.Flags().Changed() consistently — updateUser now checks all fields (including name/email) via Changed() instead of mixing empty-string checks
• Add success messages — Delete operations now confirm completion
• Add progress output to clean — Prints start/completion messages

Documentation

• Add Example fields — All subcommands now have cobra Example for --help output
• Document positional args — import/export Use strings now show [input-file]/[output-file]
• Config search order — --config flag help text now documents the full search order
• Update ARCHITECTURE.md — CMD table reflects new subcommands and flag requirements

Testing

• make lint ✅
• make client server ✅
• make test ✅ (full test suite, exit 0)

[bodji]

LGTM