refactor: Complete technical debt remediation (all steps)

Extracted TagMatcher, DownloadAssembler, DownloadBuilder, and WebhookStatus services
Slimmed dashboard.js from 1360 → 284 lines (pure HTTP layer)
Created server/routes/status.js and mounted at /api/status
Migrated frontend to vanilla ES modules under client/src/
Eliminated all tag-badge and client-logo duplication
Wired Vite build into Dockerfile and removed obsolete public/app.js
Added comprehensive DownloadBuilder regression tests

Dockerfile

@@ -9,6 +9,18 @@ WORKDIR /app
 COPY package.json package-lock.json ./
 RUN npm ci --omit=dev
 
+# ---------------------------------------------------------------------------
+# Stage 1.5 — client-build: build frontend with Vite
+# ---------------------------------------------------------------------------
+FROM node:22-alpine AS client-build
+
+WORKDIR /app/client
+
+COPY client/package.json client/package-lock.json ./
+RUN npm ci
+COPY client/ ./
+RUN npm run build
+
 # ---------------------------------------------------------------------------
 # Stage 2 — runtime image (minimal attack surface)
 # ---------------------------------------------------------------------------
@@ -33,6 +45,7 @@ COPY --from=deps /app/node_modules ./node_modules
 # Copy application source owned by root (read-only at runtime)
 COPY --chown=root:root server/ ./server/
 COPY --chown=root:root public/ ./public/
+COPY --from=client-build --chown=root:root /app/public/app.js ./public/app.js
 COPY --chown=root:root package.json ./
 # Bundled snakeoil certificate for out-of-the-box TLS (self-signed, localhost only).
 # Mount your own cert/key over /app/certs/ or set TLS_CERT/TLS_KEY env vars.