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

ui/Dockerfile

@@ -1,8 +1,13 @@
+# syntax=docker/dockerfile:1
 FROM node:22-alpine AS builder
 WORKDIR /app
 
+# Install dependencies in a separate layer so it is cached as long as
+# package-lock.json does not change.  The npm cache mount persists the
+# ~/.npm cache across builds so packages are not re-downloaded.
 COPY package.json package-lock.json ./
-RUN npm ci
+RUN --mount=type=cache,target=/root/.npm \
+    npm ci
 
 COPY . .
 
@@ -17,11 +22,19 @@ ENV PUBLIC_BUILD_COMMIT=$BUILD_COMMIT
 RUN npm run build
 
 # ── Runtime image ──────────────────────────────────────────────────────────────
-# adapter-node bundles all dependencies into build/ — no npm install needed.
+# adapter-node bundles most server-side code, but packages with dynamic
+# requires or native bindings (e.g. ioredis) are not inlined by Rollup and
+# must be present in node_modules at runtime.  We install only production
+# deps (no devDependencies) to keep the image small.
 FROM node:22-alpine
 WORKDIR /app
 
 COPY --from=builder /app/build ./build
+COPY --from=builder /app/package.json ./package.json
+COPY --from=builder /app/package-lock.json ./package-lock.json
+
+RUN --mount=type=cache,target=/root/.npm \
+    npm ci --omit=dev
 
 ENV NODE_ENV=production
 ENV PORT=3000
@@ -29,4 +42,3 @@ ENV HOST=0.0.0.0
 
 EXPOSE $PORT
 CMD ["node", "build"]
-