chore: migrate to v3, Doppler secrets, clean up legacy code

- Remove all pre-v3 code: scraper, ui-v2, backend v1, ios v1+v2, legacy CI workflows
- Flatten v3/ contents to repo root
- Add Doppler secrets management (project=libnovel, config=prd)
- Add justfile with doppler run wrappers for all docker compose commands
- Strip hardcoded env fallbacks from docker-compose.yml
- Add minimal README.md
- Clean up .gitignore

caddy/Dockerfile

@@ -0,0 +1,8 @@
+FROM caddy:2-builder AS builder
+
+RUN xcaddy build \
+    --with github.com/mholt/caddy-ratelimit \
+    --with github.com/hslatman/caddy-crowdsec-bouncer/http
+
+FROM caddy:2-alpine
+COPY --from=builder /usr/bin/caddy /usr/bin/caddy