Modernize backend README to mirror copilot-instructions and actual package config

[Copilot]

The README.MD had drifted from the repository's actual configuration and lacked an explicit map of the AI-assisted engineering workflow defined in .github/copilot-instructions.md. This rewrite realigns the docs with package.json, firebase.json, and the ESLint/Vitest configs, and adds the contributor-facing rules for human + Copilot collaboration.

Restructured into five core sections

• Project Overview & Cloud Architecture — business domain, Node.js >=22 target runtime, and how helpers integrate with Cloud Functions, Firestore, Storage, RTDB, Pub/Sub, BigQuery, and sharp.
• Tech Stack & Source Architecture Map — text-based tree of active dev directories (src/, test/, functions/src/…) plus a > [!CAUTION] alert enforcing the /lib build-output blacklist (never edit, never use as context; refresh only via npm run build).
• Local Setup & Firebase Emulation — npm install, build/build:watch, and firebase emulators:start with the exact ports from firebase.json.
• Testing & Verification Suite (Vitest) — npm test/test:watch/npx vitest, side-effect-free mocking rules (vi.mock/vi.hoisted, firebase-functions-test, emulator/stub data access), and the lint + build + test gate.
• AI-Assisted Engineering Rules — directs contributors to ground their Copilot sessions in .github/copilot-instructions.md: strict typing, Google JSDoc standards, URL preservation, async/await, thin triggers, validate-before-write.

Accuracy fixes

• Synced versions with package.json: libphonenumber-js ^1.13.5, eslint ^10.4.1, vitest ^4.1.8.
• Corrected the public API table — dropped status, which is not re-exported from src/index.ts.

Preserved

• Public API table, emulator port table, install/usage snippets, Contributing, and License sections.

Documentation-only change; no source, config, or scripts were modified.