Build container image in CI, bake in datasets, configure auth via env

- Dockerfile bakes the app and every CSV in data/ into an nginx image
- UI discovers data/*.csv via nginx JSON autoindex and offers a
  dropdown when there is more than one dataset
- docker-entrypoint.d/40-basic-auth.sh creates .htpasswd from
  BASIC_AUTH_USER / BASIC_AUTH_PASSWORD(_FILE) and refuses to start
  without credentials; replaces set-password.sh
- Gitea Actions workflow builds and pushes
  registry.traberph.de/public/interval_viz (latest, sha-*, semver)
- Escape resets the chart zoom

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docker-compose.yml

@@ -1,17 +1,10 @@
 services:
   heatpump:
-    image: nginx:1.29-alpine
+    build: .
+    image: registry.traberph.de/public/interval_viz:latest
     restart: unless-stopped
     ports:
       - "${HEATPUMP_PORT:-8080}:80"
-    volumes:
-      - ./html:/usr/share/nginx/html:ro
-      - ${HEATPUMP_CSV:-./history.csv}:/data/history.csv:ro
-      - ./nginx/default.conf:/etc/nginx/conf.d/default.conf:ro
-      - ./nginx/security-headers.conf:/etc/nginx/security-headers.conf:ro
-      - ./nginx/.htpasswd:/etc/nginx/.htpasswd:ro
-    healthcheck:
-      test: ["CMD", "wget", "-q", "-O", "/dev/null", "http://127.0.0.1/healthz"]
-      interval: 30s
-      timeout: 5s
-      retries: 3
+    environment:
+      BASIC_AUTH_USER: dev
+      BASIC_AUTH_PASSWORD: dev