refactor: move sessionSecret+passwordHash to env vars, seed data/ from defaults/

- SESSION_SECRET and PASSWORD_HASH moved out of config.json into env vars
- data.ts: AppConfig no longer holds secrets; loadConfig/loadData seed from
  defaults/ on first run if data/ files are missing
- auth.ts: requireAuth/authRequest read SESSION_SECRET from process.env directly
- login/+server.ts: reads PASSWORD_HASH and SESSION_SECRET from process.env
- defaults/config.json: ships with image (no secrets)
- defaults/units.json: ships with image as initial unit data
- package.json: add dotenv dep; start/serve load .env via -r dotenv/config
- Dockerfile: copy defaults/ into image; data/ is PVC-only
- .env.example: documents required env vars for local dev
- Remove k8s/ — managed externally

Dockerfile

@@ -9,6 +9,7 @@ FROM node:22-alpine
 WORKDIR /app
 COPY --from=builder /app/build ./build
 COPY --from=builder /app/package*.json ./
+COPY --from=builder /app/defaults ./defaults
 RUN npm ci --omit=dev --ignore-scripts
 EXPOSE 3000
 CMD ["node", "build/index.js"]