feat: OpenRouter-ready AI service — all 11 methods implemented

IAIService:
+ generateMatchExplanation, summarizeTradeOffs, generateDataQualitySummary,
  classifyMarketSignal (4 new methods covering all documented AI output types)

OpenRouterAIService:
- All 11 methods now make real API calls via withFallback() pattern
- Every fallback is explicit (console.warn/error) — no silent mock bleed-through
- Proper JSON extraction with type-safe parsers, no any casts
- parseNeed: AI JSON → ParseNeedResult mapping (no TODO stubs)
- generateFollowUpQuestions, generateMatchExplanation, summarizeTradeOffs,
  generateDataQualitySummary, classifyMarketSignal, generateOfferEmail,
  extractCriteria, generateFollowUp: fully implemented

Prompts: +followUpQuestionsPrompt, +tradeOffPrompt, +dataQualityPrompt, +marketSignalPrompt

Factory (index.ts):
- VITE_AI_PROVIDER=mock|openrouter (new, takes priority)
- VITE_USE_REAL_AI=true still supported (legacy compat)
- Missing API key → explicit console.warn + MockAIService fallback

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/services/aiService.ts

@@ -15,5 +15,12 @@ export type {
   AIServiceProvider,
   ParsedListingData,
   OfferEmailPayload,
+  MatchExplanation,
+  MatchExplanationInput,
+  TradeOffInput,
+  TradeOffSummary,
+  DataQualityInput,
+  DataQualitySummary,
+  MarketSignalClassification,
 } from './ai/IAIService'
 export { parseListingText } from './ai/mock/listingParser'