wip: Resolve Issues

[garybowers]

• Fixes api/clients: delete does not really delete a client and trying to create it again fails #103 — deleting a client now removes it from the database entirely, rather than soft-deleting it
• Recreating a client with a previously used MAC address no longer fails on the unique constraint
• Legacy soft-deleted client rows are purged on startup, so existing installations are unblocked after upgrading
• Boot logs and hardware inventory are kept but detached from the deleted client; client_images entries are removed
• Removed the now-redundant "restore soft-deleted client" behaviour from inventory saving
• Added tests covering delete-then-recreate, detachment, and the startup purge
• Fix Kali Linux Live boot (Kali Live Image does not boot because of wrong kernel files extracted #102) and add kernel/initrd override selection
• Detect Debian-style live ISOs properly: prefer /live/ kernel/initrd over the bundled debian-installer, so Kali Live no longer boots into the installer or asks for netboot files
• Detect Debian-style live ISOs properly: prefer /live/ kernel/initrd over the bundled debian-installer, so Kali Live no longer boots into the installer or asks for netboot files
• Populate and serve the squashfs path so fetch={{SQUASHFS}} resolves correctly in boot parameters
• New kernel/initrd override dropdowns in image properties — choose exactly which files from the extracted ISO are served, with an auto-detected default
• New GET /api/images/boot-candidates endpoint; PUT /api/images accepts validated kernel_override/initrd_override fields
• Overridden initrd is served with an explicit iPXE image name so initrd=initrd parameters still match
• Translations added for all five locales; unit tests for menu generation plus an opt-in integration test (BOOTIMUS_TEST_ISO)
• Verified end-to-end against kali-linux-2026.2-live-amd64.iso — extracted files are checksum-identical to the ISO's own manifest