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/Caddyfile

@@ -0,0 +1,83 @@
+# v3/Caddyfile
+#
+# Caddy reverse proxy for LibNovel v3.
+#
+# Environment variables consumed (set in docker-compose.yml):
+#   DOMAIN              — public hostname, e.g. libnovel.example.com
+#                         Use "localhost" for local dev (no TLS cert attempted).
+#   CADDY_ACME_EMAIL    — Let's Encrypt notification email (empty = no email)
+#
+# Routing rules:
+#   /health                        → backend:8080  (liveness probe)
+#   /scrape*                       → backend:8080  (scrape task creation)
+#   /api/browse                    → backend:8080  (MinIO-cached browse pages)
+#   /api/book-preview/*            → backend:8080  (live scrape, no store write)
+#   /api/chapter-text-preview/*/*  → backend:8080  (live chapter, no store write)
+#   /api/chapter-text/*/*          → backend:8080  (chapter markdown from MinIO)
+#   /api/reindex/*                 → backend:8080  (rebuild chapter index)
+#   /api/cover/*                   → backend:8080  (proxy cover image)
+#   /api/audio-proxy/*/*           → backend:8080  (proxy generated audio)
+#   /api/scrape/*                  → backend:8080  (scrape job status/tasks)
+#   /*  (everything else)          → ui:3000       (SvelteKit — handles all
+#                                                   remaining /api/* routes too)
+#
+# The SvelteKit UI itself proxies to the backend for: ranking, voices, search,
+# browse-page, presign, audio, progress, and the Go /api/progress endpoint.
+# MinIO and PocketBase are NOT exposed publicly.
+{
+	# Email for Let's Encrypt ACME account registration.
+	# Optional — omit to use an anonymous ACME account.
+	{$CADDY_ACME_EMAIL:}
+}
+
+{$DOMAIN:localhost} {
+	# ── Liveness probe ───────────────────────────────────────────────────────
+	handle /health {
+		reverse_proxy backend:8080
+	}
+
+	# ── Scrape task creation (Go backend only) ───────────────────────────────
+	handle /scrape* {
+		reverse_proxy backend:8080
+	}
+
+	# ── Backend-only API paths ────────────────────────────────────────────────
+	# These paths are served exclusively by the Go scraper and are not
+	# implemented in the SvelteKit UI.
+	handle /api/browse {
+		reverse_proxy backend:8080
+	}
+	handle /api/book-preview/* {
+		reverse_proxy backend:8080
+	}
+	handle /api/chapter-text-preview/* {
+		reverse_proxy backend:8080
+	}
+	handle /api/chapter-text/* {
+		reverse_proxy backend:8080
+	}
+	handle /api/reindex/* {
+		reverse_proxy backend:8080
+	}
+	handle /api/cover/* {
+		reverse_proxy backend:8080
+	}
+	handle /api/audio-proxy/* {
+		reverse_proxy backend:8080
+	}
+	handle /api/scrape/* {
+		reverse_proxy backend:8080
+	}
+
+	# ── SvelteKit UI (catch-all — includes all remaining /api/* routes) ───────
+	handle {
+		reverse_proxy ui:3000
+	}
+
+	# ── Logging ──────────────────────────────────────────────────────────────
+	log {
+		output stdout
+		format json
+	}
+}
+}