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/ai-reply.blade.php

@@ -0,0 +1,26 @@
+@php
+    $isError = isset($meta['error']);
+    $provider = $meta['provider'] ?? null;
+    $tokensIn = $meta['tokens_in'] ?? null;
+    $tokensOut = $meta['tokens_out'] ?? null;
+    $latency = $meta['latency_ms'] ?? null;
+@endphp
+<div class="space-y-3">
+    @if ($isError)
+        <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">
+            {{ $reply }}
+        </div>
+    @else
+        <div class="prose prose-sm dark:prose-invert max-w-none whitespace-pre-wrap leading-relaxed text-gray-900 dark:text-gray-100">{{ $reply }}</div>
+    @endif
+
+    @if (! $isError && ($provider || $tokensIn !== null))
+        <div class="text-xs text-gray-500 dark:text-gray-400 border-t border-gray-200 dark:border-gray-700 pt-2 flex flex-wrap gap-3">
+            @if ($provider) <span>Provider: <strong>{{ strtoupper($provider) }}</strong></span> @endif
+            @if ($tokensIn !== null || $tokensOut !== null)
+                <span>Tokens: <strong>{{ ($tokensIn ?? 0) + ($tokensOut ?? 0) }}</strong> ({{ $tokensIn ?? 0 }} in / {{ $tokensOut ?? 0 }} out)</span>
+            @endif
+            @if ($latency !== null) <span>Latency: {{ $latency }} ms</span> @endif
+        </div>
+    @endif
+</div>