feat(timetable): seven artist-domain services + supporting exceptions

GenreService, ArtistService, ArtistEngagementService (state machine),
StageService, StageDayService, PerformanceService, LaneCascadeService
under app/Services/Artist/. Plain final classes with constructor
injection — matches FormSubmissionService convention.

ArtistEngagementService implements the RFC §10.1 booking_status state
machine: terminal Cancelled/Rejected/Declined, Option requires future
option_expires_at, Contracted requires fee_amount > 0. transitionStatus
is the focused entry point; update() routes through it whenever the
payload mutates booking_status. cancel() composes transitionStatus +
soft delete in one transaction so the existing
ArtistEngagementObserver cascade fires.

LaneCascadeService is the D18 transactional move algorithm. Locks the
dragged Performance row FOR UPDATE, validates client version against
the persisted version (D14), then either parks (stage_id=null, no
cascade) or places onto (stage, event, lane) with single-level
cascade-bump of any time-overlapping rows on the target lane. Returns
a MoveResult value object carrying the moved + cascaded performances
so the controller maps them to API resources without a second query.

StageDayService implements the §10.5 atomic matrix replace. Detects
non-cancelled performances on event_ids about to be removed; throws
StageDaysOrphanedPerformancesException unless force_orphan=true. The
orphans are not deleted — they persist with the same stage_id so they
re-appear when the day re-activates (D5/D27 retention).

ArtistService.create raises DuplicateArtistException carrying the
existing master so the controller can offer a "use existing" choice
instead of forcing the booker to abandon their dialog. ArtistEngagement
defaults buma_handled_by based on artist.agent_company.handles_buma
per RFC D26.

GenreService.delete is hard-blocked (GenreInUseException) when artists
still reference the genre via default_genre_id; the frontend rebinds
those artists first.

StageService.delete cascade-parks performances (stage_id → null, lane
preserved) and returns the parked count for the activity-log entry
the controller writes in Step 9.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/app/Exceptions/Artist/DuplicateArtistException.php

@@ -0,0 +1,24 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\Artist;
+
+use App\Models\Artist;
+use DomainException;
+
+/**
+ * Raised by ArtistService::create when an exact-name match (case-insensitive)
+ * already exists in the same organisation. The handler can surface the
+ * existing artist's id to the UI so a "use existing or rename" choice is
+ * presented to the booker.
+ */
+final class DuplicateArtistException extends DomainException
+{
+    public function __construct(
+        public readonly Artist $existing,
+        string $message = 'An artist with this name already exists in this organisation.',
+    ) {
+        parent::__construct($message);
+    }
+}