store: reject create conflicts

[aberoham]

create() with an id that already existed returned the existing row. 201 Created, the caller's payload silently discarded, and a response body describing a record they never wrote.

POST /zone_record {id: 5196, owner: 'collide.autoinc.example.com.', ...}
-> 201 Created
-> id 5196, owner victim.autoinc.example.com.

The short-circuit was deliberate: suites re-create fixtures by fixed id without cleaning up first. That's now something a caller asks for, create(args, { ifExists: 'return' }), and only the test helpers ask. Everything else gets a 409.

Every store has the pattern, mysql and file alike, so all of them changed and the backends still agree.

A second bug lives in the same lines and gets the same fix: lib/permission/store/mysql.js flattened the nested {user:{id}} payload after looking for an existing row, so two POSTs wrote two nt_perm rows. objectToDb() now runs first, and the check covers user rows as well as group rows.

mysql  404 tests, 404 pass, 0 failed, 0 skipped
json   381 tests, 381 pass, 0 failed, 0 skipped
toml   381 tests, 381 pass, 0 failed, 0 skipped