Docs: Add service-level documentation & contributor guidance for AccountsService (#49)

[OthmanImam]

Closes #49

Changes

• Added a comprehensive top‑level comment block to AccountsService explaining
  its role in the Bridgelet flow.
• Documented lifecycle steps (creation, funding, claim), security assumptions,
  integration boundaries, DTO/response‑mapping intent, and MVP placeholders.
• Inserted inline “security notes” and “ask if unsure” guidance throughout the
  file.
• No runtime logic modified; all additions are comments for maintainers.

Testing

✅ Lint passed
✅ Formatting (npm run format:check) passes after formatting
✅ All 46 unit tests pass locally
✅ Build completes without errors

[OthmanImam]

@phertyameen