Stage 16 — AI Layer: VIN decoder + diagnostic / parts / price helpers

VinDecoder (deterministic, no API):
- ISO 3779/3780 parsing: WMI manufacturer (~60 brands), year (cyclical with
  post-2010 disambiguation via position 7), region, plant, NA checksum
- Strip non-VIN chars, accept dashes/spaces, reject I/O/Q per spec

AiAssistantService:
- Refactored provider HTTP into postClaude/postOpenAI/postGemini so both
  chat history and one-shot calls share the same transport
- singleShot(system, userPrompt, provider?) for fire-and-forget calls
- 4 specialized helpers with tight prompts:
  - suggestDiagnosis(WO) — diagnostician based on complaint + VIN info
  - suggestParts(WO, task) — OEM parts list for an operation
  - suggestPrice(Part) — markup recommendation with justification
  - vinRecommendations(vin, mileage) — scheduled maintenance from decoded VIN
- monthlyUsage() — token spend MTD by provider

Filament:
- VehicleResource: "Decode VIN" + "AI: recomandări" actions
- WorkOrderResource Edit: "AI: sugerează diagnostic" header action
- PartResource: "AI: preț recomandat" action
- Shared views: filament.tenant.ai-reply, filament.tenant.vin-decode
- AiAssistant page shows monthly token usage banner

Tests (13 new):
- 8 VinDecoder unit tests with real VIN samples (Honda 2003, VW 1999, Audi
  2014, Dacia, unknown WMI, lowercase/dashes, forbidden chars)
- 5 AiHelpers feature tests with Http::fake covering all providers + no-key
  fallback + token usage aggregation

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

resources/views/filament/tenant/vin-decode.blade.php

@@ -0,0 +1,43 @@
+@php
+    $valid = $info['valid_length'] ?? false;
+@endphp
+<div class="space-y-4">
+    @if (! $valid)
+        <div class="p-3 rounded-md bg-yellow-50 dark:bg-yellow-900/30 border border-yellow-200 dark:border-yellow-700 text-sm text-yellow-900 dark:text-yellow-100">
+            {{ $info['reason'] ?? 'VIN invalid.' }}
+        </div>
+    @else
+        <dl class="grid grid-cols-2 gap-x-4 gap-y-2 text-sm">
+            <dt class="text-gray-500 dark:text-gray-400">VIN</dt>
+            <dd class="font-mono text-gray-900 dark:text-gray-100">{{ $info['vin'] }}</dd>
+
+            <dt class="text-gray-500 dark:text-gray-400">Regiune</dt>
+            <dd class="text-gray-900 dark:text-gray-100">{{ $info['region'] ?? '—' }}</dd>
+
+            <dt class="text-gray-500 dark:text-gray-400">Țară</dt>
+            <dd class="text-gray-900 dark:text-gray-100">{{ $info['country'] ?? '—' }}</dd>
+
+            <dt class="text-gray-500 dark:text-gray-400">Producător</dt>
+            <dd class="text-gray-900 dark:text-gray-100 font-semibold">{{ $info['manufacturer'] ?? '— (WMI necunoscut)' }}</dd>
+
+            <dt class="text-gray-500 dark:text-gray-400">An model</dt>
+            <dd class="text-gray-900 dark:text-gray-100 font-semibold">{{ $info['year'] ?? '—' }}</dd>
+
+            <dt class="text-gray-500 dark:text-gray-400">Cod uzină</dt>
+            <dd class="font-mono text-gray-900 dark:text-gray-100">{{ $info['plant'] ?? '—' }}</dd>
+
+            <dt class="text-gray-500 dark:text-gray-400">Checksum (ISO)</dt>
+            <dd class="text-gray-900 dark:text-gray-100">
+                @if ($info['checksum_valid'])
+                    <span class="text-green-600 dark:text-green-400">✓ valid</span>
+                @else
+                    <span class="text-gray-500">— (multe VIN-uri europene nu respectă checksum-ul NA)</span>
+                @endif
+            </dd>
+        </dl>
+
+        <div class="text-xs text-gray-500 dark:text-gray-400 border-t border-gray-200 dark:border-gray-700 pt-2">
+            Detalii granulare (model, motorizare) necesită bază TecDoc/NHTSA. Folosește butonul „AI: recomandări" pentru sugestii bazate pe an + producător.
+        </div>
+    @endif
+</div>