feat: unified error handling + AI service modularisation

Error handling (Prompt 2):
- src/services/errors.ts: AppError class, normalizeError(), throwServiceError() helper
- 6 services wrapped with try/catch (property, match, need, shortlist, futureSignal, inquiry)
- inquiryService aligned from custom ServiceResult<T> to standard ServiceResponse types
- Results, MatchCenter, FutureAvailability pages show <ErrorState onRetry> on query failure

AI modularisation (Prompt 3):
- src/services/aiService.ts reduced from 755 → 19 lines (barrel re-export)
- src/services/ai/IAIService.ts: typed interface + all response types
- src/services/ai/mock/: needParser, compareBuilder, decisionBrief, listingParser, MockAIService
- src/services/ai/openrouter/OpenRouterAIService.ts: model-agnostic skeleton
- src/services/ai/prompts/: 4 prompt template files (needParsing, matchExplanation, compareSummary, decisionBrief)
- src/services/ai/index.ts: factory selects Mock or OpenRouter via VITE_USE_REAL_AI flag
- All existing import paths unchanged — zero call-site modifications

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

src/pages/demand/Results.tsx

@@ -13,6 +13,7 @@ import {
   UnifiedResultFeed,
 } from '../../components/results'
 import { AddToPipelineDialog } from '../../components/shortlist'
+import { ErrorState } from '../../components/ui'
 import { useSessionStore } from '../../stores/sessionStore'
 import type { UnifiedMatchResult } from '../../domain/unifiedResult'
 
@@ -65,7 +66,7 @@ export default function Results() {
     queryClient.invalidateQueries({ queryKey: ['needs'] })
   }
 
-  const { data: results = [], isLoading } = useUnifiedResults(effectiveNeedId)
+  const { data: results = [], isLoading, error } = useUnifiedResults(effectiveNeedId)
 
   const isStaff = currentUser?.role === 'PROPERTY_MANAGER' || currentUser?.role === 'ORGANIZATION_ADMIN'
 
@@ -185,6 +186,8 @@ export default function Results() {
 
         {isLoading ? (
           <FeedSkeleton />
+        ) : error ? (
+          <ErrorState message={(error as Error).message} />
         ) : sorted.length === 0 ? (
           <FeedEmptyState filtered={filterSource !== 'ALL' || !showFutureAvailability || !showOwnProperties} />
         ) : (