fix(compose): wire all documented backend env vars + regression test (0.87.12)

0.87.1 added 12+ env vars to `.env.example` but the compose backend
`environment:` block forwarded none of them — `.env` interpolation
doesn't pass vars to the container. Operators got
anonymous-serving sites, silent ADMIN_* no-ops, and missing
ANALYSIS_* configuration.

Wire 36 backend-consumed vars; also substitute the hardcoded
`BACKEND_URL` on the frontend; emit a startup warning on half-set
`ADMIN_USERNAME`/`ADMIN_PASSWORD`. Regression-test parses
docker-compose.yml and asserts the wiring.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.87.11",
+  "version": "0.87.12",
   "private": true,
   "type": "module",
   "scripts": {