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

.gitignore

@@ -5,16 +5,16 @@
 /dist/
 
 # ── Compiled binaries ──────────────────────────────────────────────────────────
-scraper/bin/
-scraper/scraper
+backend/bin/
 
-# ── Scraped output (large, machine-generated) ──────────────────────────────────
-
-/static/books
 # ── Environment & secrets ──────────────────────────────────────────────────────
+# Secrets are managed by Doppler — never commit .env files.
 .env
 .env.*
-!.env.example
+.env.local
+
+# ── CrowdSec — generated bouncer API key ──────────────────────────────────────
+crowdsec/.crowdsec.env
 
 # ── OS artefacts ───────────────────────────────────────────────────────────────
 .DS_Store