feat(marketing): bootstrap Tailwind v4 + design tokens FlexiHub

Adds Tailwind v4 / PostCSS pipeline that compiles to static/css/marketing.css,
to be loaded only by future templates/marketing/** templates and to coexist
with the existing legacy v3 JIT runtime used by index.html / account.html /
admin.html. The legacy v3 runtime stays untouched.

- package.json: postcss-cli build:css and watch:css scripts
- postcss.config.js: @tailwindcss/postcss + autoprefixer
- static/css/tailwind.config.js: brand tokens (b1/b2/b3, navy, navy2, navy3,
  bg, border), font families, brand-grad, cta shadows, FlexiHub keyframes
  (tc-fade-in-up/right, tc-float-y, tc-pulse-glow, plus-breathe). content
  paths scoped to marketing/legal/billing/macros only - purge cannot touch
  legacy templates.
- static/css/input.css: @import "tailwindcss"; + @config directive (Tailwind
  v4 backward-compat for v3-style JS config). @font-face Inter Variable +
  JetBrains Mono Variable (woff2). base layer body font/color, h1-h3
  letter-spacing. utilities: grad-text, grad-bg, eyebrow.
- Dockerfile: new stage 3 'assets-builder' (node:20-alpine) compiles CSS;
  runtime stage copies the built file in via --from=assets-builder, after
  COPY . . so the freshly-built file always wins.
- .gitignore + .dockerignore: exclude node_modules.

Build verified locally: marketing.css = 121 KB minified (Tailwind v4 ships
all default theme tokens + reset properties even with empty content;
realistic baseline, will not grow much as marketing markup is added).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -43,7 +43,17 @@ RUN apt-get update && apt-get install -y --no-install-recommends wget xz-utils \
     && rm -rf /tmp/ff.tar.xz /tmp/ffmpeg-dir
 
 ###############################################################################
-# Stage 3: Runtime — lean final image with only what's needed
+# Stage 3: Assets builder — compile Tailwind v4 marketing CSS
+###############################################################################
+FROM node:20-alpine AS assets-builder
+WORKDIR /app
+COPY package.json postcss.config.js ./
+COPY static/css ./static/css
+COPY templates ./templates
+RUN npm install --no-audit --no-fund && npm run build:css
+
+###############################################################################
+# Stage 4: Runtime — lean final image with only what's needed
 ###############################################################################
 FROM python:3.11-slim
 
@@ -62,6 +72,9 @@ COPY --from=builder /app/static/vendor /app/static/vendor
 # Copy application code
 COPY . .
 
+# Copy compiled marketing CSS (overrides any stale file from `COPY . .`)
+COPY --from=assets-builder /app/static/css/marketing.css /app/static/css/marketing.css
+
 # Create necessary directories
 RUN mkdir -p /data/uploads /data/instance && chmod 755 /data/uploads /data/instance