docs: update README, env example, and design docs

- backend/.env.example: add GEMINI_API_KEY and PEXELS_API_KEY placeholders
- backend/Makefile: add test-integration to PHONY targets
- backend/README.md: document external API keys, import/translate commands
- docs/Design.md, docs/Tech.md: reflect Iteration 1 implementation and future plans

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

backend/.env.example

@@ -12,3 +12,7 @@ JWT_REFRESH_DURATION=720h
 # Server
 PORT=8080
 ALLOWED_ORIGINS=http://localhost:3000
+
+# External APIs
+GEMINI_API_KEY=your-gemini-key
+PEXELS_API_KEY=your-pexels-key