is_conflict is too narrow, so an import does not converge on replay

[jorisvilardell]

Describe the bug

is_conflict (libs/ferriskey-cli-core/src/import/apply.rs) recognises a conflict only from a 409, or a 400 whose body contains exist. The server answers 500 when a realm name is already taken (a raw realms_name_key unique violation), so re-running an import stops at its first line instead of converging — while the file header promises that "an import can be replayed to converge".

Expected behaviour

Replaying an import must converge. Two changes, either of which helps on its own:

1. Widen is_conflict to recognise a unique-constraint violation surfaced as a 500 (the body names the constraint), so the CLI stays usable against servers already deployed.
2. Have the server answer 409 — tracked in ferriskey/ferriskey.

Point 1 should not be dropped once point 2 ships: the CLI has to keep working against older servers.

A skipped conflict should be reported in ImportReport as "already present", so a converging run is distinguishable from one that did nothing.

Environment

ferris-ctl 0.2.3, FerrisKey server 0.7.0

[jorisvilardell]

Server-side half: ferriskey/ferriskey#1286 (POST /realms answers 500 instead of 409 on a duplicate name).