fix(content): repair Django tutorial crash and complete the hardening sweep

Follow-ups to the 9 secret-hardening commits (each verified against
docs.docker.com / vendor docs; one outright new bug found and fixed):

- 11.6: eb5e4397 changed settings.py to a hard os.environ lookup but the
  web service never receives POSTGRES_PASSWORD (only db does; DATABASE_URL
  is set but never read) -> step-6 'docker compose up' crashed with
  KeyError. Pass the var to web; harden the leftover literal password in
  the 配置详解 snippet that contradicted the new guidance two lines down;
  blank line after the inserted sentence (bold heading merged into the
  paragraph); dev/prod table no longer claims dev uses 明文 passwords
- 11.8: FAQ still told readers to check passwords in .env after the same
  file banned passwords in .env -> point at secrets/db_password.txt;
  backup sidecar env vars updated to tiredofit/db-backup 4.x interface
  (DB01_* + DB01_PASS_FILE + DB01_BACKUP_INTERVAL - the unprefixed DB_*
  names are ignored by current :latest; verified against upstream README)
- demo/wordpress: compose now references secrets files that ship nowhere
  -> add README with the creation commands from 11.8; demo/django: align
  with the hardened 11.6 (env-injected password, passed to web too)
- 04_image multistage demos: go build without go.mod fails on module-mode
  Go (reproduced by reviewer on go1.26) -> add 'go mod init helloworld'
  matching the 7.17 doc pattern in all three Dockerfiles
- 21.7: init script reworked init-db.sql -> init-db.sh per the official
  image's env-reading .sh hook - removes the baked 'secure_password' AND
  the CREATE DATABASE myappdb collision with POSTGRES_DB that aborted
  first-boot init (ON_ERROR_STOP); compose passes APP_DB_PASSWORD;
  microservices init.sql mount annotated schema-only (POSTGRES_USER:
  appuser would collide with CREATE USER); Dockerfile-redis healthcheck
  now authenticates via REDISCLI_AUTH read from redis.conf (plain
  redis-cli ping gets NOAUTH against requirepass - same class as the
  compose sibling eb5e4397 already fixed); dev-container dev/dev creds
  annotated local-only
- 19.3: Grafana admin password 'admin' sat directly under the newly added
  security warning -> env-injected like the rest of the stack

19_observability/19.3_performance_optimization.md

@@ -229,7 +229,7 @@ services:
     ports:
       - "3000:3000"
     environment:
-      - GF_SECURITY_ADMIN_PASSWORD=admin
+      - GF_SECURITY_ADMIN_PASSWORD=${GRAFANA_ADMIN_PASSWORD:?set GRAFANA_ADMIN_PASSWORD}
       - GF_INSTALL_PLUGINS=grafana-piechart-panel
     volumes:
       - grafana_data:/var/lib/grafana