feat(v3): add v3 stack — backend rewrite, renamed env vars, docs

- New Go backend binary (backend + runner) replacing old scraper/
- Rename SCRAPER_API_URL → BACKEND_API_URL in UI env and docker-compose
- Rename scraperFetch → backendFetch across all 19 UI server files
- Remove SCRAPER_PROXY env var and proxy transport from browser.Config
- Add Meilisearch, Valkey, Caddy to docker-compose
- Add docs/: api-endpoints.md, request-flow.mermaid.md, data-flow.mermaid.md

v3/backend/go.mod

@@ -0,0 +1,36 @@
+module github.com/libnovel/backend
+
+go 1.26.1
+
+require (
+	github.com/minio/minio-go/v7 v7.0.98
+	golang.org/x/net v0.51.0
+)
+
+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/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
+	golang.org/x/text v0.34.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)