refactor: improve stability, boundaries, and delivery

[LQ458]

Summary

• add production characterization tests, health/readiness boundaries, authorization checks, and graceful startup/shutdown
• separate backend services and centralize frontend data access without changing the database contract
• add lightweight multilingual catalog search and responsive media delivery while preserving original files and legacy URLs
• add privacy-safe operational evidence, deployment/rollback documentation, and public repository guidance
• include the verified frontend API-routing recovery and global button-style restoration

Validation

• npm run verify
• backend: 39/39 tests passed
• frontend: 19/19 tests passed
• production build succeeded with the existing recorded CRA, Browserslist, and ESLint warnings
• public root and readiness are HTTP 200; production acceptance and rollback evidence are recorded under docs/operations/

Compatibility and deployment

• no MongoDB schema migration or destructive media rewrite
• existing API and legacy media paths remain supported
• production data, Redis, media, environment values, and Nginx are outside this Git merge

Known follow-up risks

• no independent staging environment or completed restore rehearsal
• existing development dependency findings and CRA/ESLint warnings remain documented
• visits, verified unique visitors, historical uptime, and other unsupported metrics remain explicitly unavailable