feat(backend): add POST /api/meters/batch-register accepting array of {meter_id, owner} pairs

[Dev-AdeTutu]

Summary

Registering many meters one at a time via POST /api/meters is slow. Add a batch endpoint that submits multiple registrations and returns per-item results so bulk onboarding does not require N sequential HTTP calls.

Acceptance Criteria

• POST /api/meters/batch accepts { meters: [{ meter_id, owner }] } (max 50 items)
• Returns { results: [{ meter_id, hash?, error? }] } — partial success allowed
• Each item is validated individually; invalid items are skipped with an error message
• Protected by requireAdminKey

Files to touch

• backend/src/routes/meters.ts