fix(search): let the searching user pick themselves as attendee

[luflow]

Summary

An organizer could not add themselves as an attendee of their own appointment: the audience picker never offered them, neither when creating nor when editing later.

• Root cause: the users/groups/teams picker is built on Nextcloud's sharee search, whose UserPlugin unconditionally removes the searching user (nobody shares with themselves). The organizer picker only hid this by prefilling the current user client-side.
• Why it hurts since 1.54.0: only the audience may answer an appointment ([BUG] Response handling of Organizers who are not part of limited access group(s) for an event #251). A sole organizer who invites individuals is therefore locked out of responding — the web shows the "not an attendee" hint, mobile builds before 1.9.0 surface the server error.
• Fix: the search puts the searching user back when the query fits their user ID, display name or email (case-insensitive, substring). An exact hit leads the list like the sharee search's own exact matches, a partial hit closes it, and a user the sharee search already lists is not duplicated.
• The entry shape moves to ResponseDefinitions as AttendanceDirectoryEntry; the generated specs now carry it as a named DirectoryEntry component (same shape as before).

The mobile app calls the same endpoint, so it gets the fix with the server update — no app change and no capability flag needed, the response only gains one more entry of an existing shape.

Second commit: DirectorySearchService

The search cluster depended on nothing appointment-related, yet lived in the 2600-line AppointmentService with its tests wired through that class's two dozen mocks. It now has its own service and a focused test class; AppointmentController is the only caller and keeps the permission gate as every sibling action does. The move is behaviour-neutral and sheds carried-over baggage (a dead ISearchResult branch, an unused $hasMore, comments restating the code). Psalm baseline entries that still apply moved with the code by hand; the ones the typing fixed are gone.

Test plan

• ./scripts/check.sh green on both commits (eslint, stylelint, php-cs-fixer, psalm, PHPUnit, vite build, l10n, OpenAPI)
• Six unit tests in DirectorySearchServiceTest: partial hit on the user ID (appended), exact hit on the display name (leads), hit on the email, no hit, no duplicate, unknown searcher
• Live in the stable34 dev container as admin: ?search=admin returned only group:admin before and user:admin, group:admin after; ?search=adm appends user:admin

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