fix: address remaining code review issues

- Apply sanitizeForAIPrompt to AI prompt (prevent prompt injection)
- Replace hardcoded provider IDs with name-based filtering
- Remove dead code (queryVPSBenchmarks function)
- Use LIMITS.MAX_REQUEST_BODY_BYTES constant
- Change parseAIResponse parameter from `any` to `unknown`

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

REFACTOR_SUMMARY.md

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+# Modularization Summary
+
+## New File Structure
+
+```
+src/
+├── index.ts                  # Main worker entry point (clean, ~90 lines)
+├── types.ts                  # All TypeScript interfaces (complete)
+├── config.ts                 # USE_CASE_CONFIGS and i18n messages (complete)
+├── utils.ts                  # Utility functions (~650 lines, complete)
+├── handlers/
+│   ├── health.ts            # GET /api/health (complete)
+│   ├── servers.ts           # GET /api/servers (complete)
+│   └── recommend.ts         # POST /api/recommend (~1100 lines)
+```
+
+## Functions by Module
+
+### index.ts (NEW)
+- Main fetch handler
+- Route handling
+- CORS setup
+- Rate limit checking
+
+### types.ts (COMPLETE)
+All interfaces extracted from original index.ts
+
+### config.ts (COMPLETE)
+- USE_CASE_CONFIGS
+- i18n messages
+
+### utils.ts (COMPLETE)
+- jsonResponse()
+- hashString()
+- sanitizeCacheValue()
+- generateCacheKey()
+- escapeLikePattern()
+- Type guards: isValidServer(), isValidVPSBenchmark(), etc.
+- validateRecommendRequest()
+- getAllowedOrigin()
+- findUseCaseConfig()
+- getDauMultiplier()
+- getActiveUserRatio()
+- estimateBandwidth()
+- getProviderBandwidthAllocation()
+- calculateBandwidthInfo()
+- checkRateLimit()
+
+### handlers/health.ts (COMPLETE)
+- handleHealth()
+
+### handlers/servers.ts (COMPLETE)
+- handleGetServers()
+
+### handlers/recommend.ts (TO CREATE)
+- handleRecommend() - Main handler
+- queryCandidateServers() - D1 query for servers
+- queryBenchmarkData() - Phoronix benchmarks
+- getBenchmarkReference() - Match benchmarks to servers
+- queryVPSBenchmarks() - Geekbench data
+- queryVPSBenchmarksBatch() - Batch benchmark query
+- formatVPSBenchmarkSummary() - Format for AI prompt
+- formatBenchmarkSummary() - Format for AI prompt
+- queryTechSpecs() - Tech stack specs
+- formatTechSpecsForPrompt() - Format for AI prompt
+- getAIRecommendations() - OpenAI GPT-4o-mini call
+- parseAIResponse() - Parse AI JSON response
+
+## Migration Status
+
+✅ types.ts - DONE
+✅ config.ts - DONE
+✅ utils.ts - DONE
+✅ handlers/health.ts - DONE
+✅ handlers/servers.ts - DONE
+⏳ handlers/recommend.ts - IN PROGRESS (extracted functions ready)
+⏳ index.ts - NEW version created (index.new.ts)
+
+## Next Steps
+
+1. Create handlers/recommend.ts with all extracted functions
+2. Backup original index.ts
+3. Replace index.ts with index.new.ts
+4. Run TypeScript typecheck
+5. Test with `npm run dev`
+
+## Size Reduction
+
+Original: index.ts (2381 lines)
+New distribution:
+- index.ts: ~90 lines (97% reduction)
+- types.ts: ~175 lines
+- config.ts: ~140 lines
+- utils.ts: ~650 lines
+- handlers/: ~1300 lines total
+
+Total: ~2355 lines (modular + maintainable)