Show lease time of reservations in the UI

[romanlum]

Summary

• Replace the Expiry == -1 sentinel for DHCP reservations with an explicit reservation boolean field on leases, so a reservation's actual last-issued lease time can still be tracked and shown instead of being overwritten by the sentinel.
• Reservations no longer get an etcd TTL lease attached (so their record never expires), but Expiry is still updated on each Put for informational display; legacy records with expiry: -1 are migrated to reservation: true on read.
• Update the MS DHCP import converter and the web UI (lease form + leases table) to use the new reservation field, and show "last leased until" for reservations that have an actual expiry.

Test plan

• pkg/roles/dhcp: added unit tests for IsReservation(), legacy expiry: -1 migration in leaseFromKV, and that Put/CreateLeaseIfAbsent skip granting an etcd TTL lease for reservations while still tracking Expiry.
• pkg/roles/dhcp: added API-level tests (APILeasesPut/APILeasesGet) verifying the reservation field round-trips through the API.
• go test -p 1 ./pkg/roles/dhcp/... ./pkg/convert/... passes locally against etcd.
• [x ] Manual check of the DHCP leases page in the browser (create/mark a reservation, verify "last leased until" renders).

🤖 Generated with the help of Claude Code

[netlify]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.81%. Comparing base (c311230) to head (3ced8b3).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2317      +/-   ##
==========================================
+ Coverage   69.79%   69.81%   +0.02%     
==========================================
  Files         140      140              
  Lines        7733     7739       +6     
==========================================
+ Hits         5397     5403       +6     
  Misses       1794     1794              
  Partials      542      542              

Flag	Coverage Δ
bench	12.72% <45.00%> (+0.01%)	⬆️
e2e	45.13% <50.00%> (+<0.01%)	⬆️
unittest	56.03% <100.00%> (+0.18%)	⬆️

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