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/pages/ai-assistant.blade.php

@@ -86,8 +86,21 @@
     @php
         $chat = $this->getChat();
         $chats = $this->getChats();
+        $usage = $this->getUsage();
+        $totalTokens = collect($usage)->sum(fn ($u) => $u['tokens_in'] + $u['tokens_out']);
+        $totalCalls = collect($usage)->sum('calls');
     @endphp
 
+    @if ($totalCalls > 0)
+        <div style="margin-bottom:12px;padding:10px 14px;background:#eff6ff;border:1px solid #bfdbfe;border-radius:8px;font-size:12px;color:#1e3a8a;">
+            <strong>Tokens consumate luna aceasta:</strong>
+            {{ number_format($totalTokens) }} tokens · {{ $totalCalls }} cereri
+            @foreach ($usage as $provider => $u)
+                · {{ strtoupper($provider) }}: {{ number_format($u['tokens_in'] + $u['tokens_out']) }}
+            @endforeach
+        </div>
+    @endif
+
     <div class="ai-wrap" x-data x-init="$nextTick(() => { const m = $refs.msgs; if (m) m.scrollTop = m.scrollHeight; })">
         {{-- Sidebar: chat history --}}
         <div class="ai-side">