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

backend/go.mod

@@ -8,19 +8,27 @@ require (
 )
 
 require (
+	github.com/andybalholm/brotli v1.1.1 // indirect
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/dgryski/go-rendezvous v0.0.0-20200823014737-9f7001d12a5f // indirect
 	github.com/dustin/go-humanize v1.0.1 // indirect
+	github.com/getsentry/sentry-go v0.43.0 // indirect
 	github.com/go-ini/ini v1.67.0 // indirect
+	github.com/golang-jwt/jwt/v5 v5.3.1 // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/klauspost/compress v1.18.2 // indirect
 	github.com/klauspost/cpuid/v2 v2.2.11 // indirect
 	github.com/klauspost/crc32 v1.3.0 // indirect
+	github.com/meilisearch/meilisearch-go v0.36.1 // indirect
 	github.com/minio/crc64nvme v1.1.1 // indirect
 	github.com/minio/md5-simd v1.1.2 // indirect
 	github.com/philhofer/fwd v1.2.0 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/redis/go-redis/v9 v9.18.0 // indirect
 	github.com/rs/xid v1.6.0 // indirect
 	github.com/tinylib/msgp v1.6.1 // indirect
+	go.uber.org/atomic v1.11.0 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
 	golang.org/x/crypto v0.48.0 // indirect
 	golang.org/x/sys v0.41.0 // indirect