Fix LOW findings from 2026-09-24 maintenance review

- Frontend on nginx-unprivileged (non-root, container port 8080)
- Replace unmaintained passlib with bcrypt 5.0.0 (hash-compatible)
- Pass DOCS_ENABLED through to the backend container
- Raise db mem_limit to 768m for MySQL 8.4

Co-Authored-By: Claude Opus 5.5 <[email protected]>

backend/requirements.txt

@@ -7,8 +7,7 @@ aiomysql==0.3.0
 PyMySQL==1.1.2
 PyJWT==2.15.0
 cryptography==50.0.1
-passlib[bcrypt]==1.7.4
-bcrypt==3.2.2
+bcrypt==5.0.0
 pydantic==2.13.5
 pydantic-settings==2.5.2
 alembic==1.20.0