docs(legibility): fix three review blockers in DOC-7

- docs/README.md: remove duplicate infrastructure/ entry at end of folder tree
- ocr-service/CLAUDE.md: add **LLM reminder:** prefix to ALLOWED_PDF_HOSTS
  SSRF warning (consistent with all other machine-readable instructions)
- backend/CLAUDE.md: restore ResponseStatusException note for simple controller
  validation — avoids LLMs reaching for DomainException for trivial checks

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

docs/README.md

@@ -14,8 +14,7 @@ docs/
 ├── DEPLOYMENT.md            # Day-1 checklist and operational reference (DOC-5)
 ├── GLOSSARY.md              # Domain terminology (DOC-3)
 ├── security-guide.md        # Security policies and hardening guide
-├── STYLEGUIDE.md            # Coding and design style guide
-└── infrastructure/          # Production compose, CI config, S3 migration
+└── STYLEGUIDE.md            # Coding and design style guide
 ```
 
 ## ADR (`adr/`)