chore(backend): add env config template

Scaffolding only — FastAPI app, SQLAlchemy models, Alembic migrations,
and docker-compose from the original task description are still not
implemented. Committing what exists so it isn't lost; task ec0074121ca3
reset to pending to finish the rest.

backend/.env.example

@@ -0,0 +1,14 @@
+# Database
+DATABASE_URL=postgresql://agp_user:agp_password@localhost:5432/agp_db
+
+# Redis
+REDIS_URL=redis://localhost:6379/0
+
+# JWT
+SECRET_KEY=change-me-to-a-secure-random-string
+ALGORITHM=HS256
+ACCESS_TOKEN_EXPIRE_MINUTES=30
+
+# App
+DEBUG=true
+CORS_ORIGINS=http://localhost:8080,http://localhost:3000