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

README.md

@@ -0,0 +1,38 @@
+# LibNovel
+
+Self-hosted audiobook platform. Go backend + SvelteKit UI + MinIO/PocketBase/Meilisearch.
+
+## Requirements
+
+- Docker + Docker Compose
+- [just](https://github.com/casey/just)
+- [Doppler CLI](https://docs.doppler.com/docs/install-cli)
+
+## Setup
+
+```sh
+doppler login
+doppler setup   # project=libnovel, config=prd
+```
+
+## Usage
+
+```sh
+just up          # start everything
+just down        # stop
+just logs        # tail all logs
+just log backend # tail one service
+just build       # rebuild images
+just restart     # down + up
+just secrets     # view/edit secrets
+```
+
+## Secrets
+
+Managed via Doppler (`project=libnovel`, `config=prd`). No `.env` files.
+
+To add or update a secret:
+
+```sh
+doppler secrets set MY_SECRET=value
+```