feat: list user passkeys (#640)

## Summary

- Added list passkeys endpoint

## Validation

- `go run ./... -tags postgres_integration`

## Release notes / changeset

- Changeset: .changeset/list-user-passkeys.md

## Notes

---------

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: wim07101993

.changeset/list-user-passkeys.md

@@ -0,0 +1,14 @@
+---
+"@zitadel/server": minor
+"@zitadel/api": minor
+---
+
+Add `GET /users/{user_id}/passkeys` to list a user's registered passkeys, 
+returning each passkey's `id`, `name`, and `created_at`. Requires an OAuth2 
+bearer with the `user.read` scope.
+
+Registered passkeys now get a name of their own instead of reusing the user's 
+display name, which was the same for every passkey a user registered (and empty 
+whenever the flow collected no identifier). A passkey takes the name the 
+registering caller supplies, and otherwise one derived from the credential 
+itself: `Security key`, `Synced passkey`, or `Device-bound passkey`.