feat: REST user billing history (transactions)

[precious-akpan]

Implements GET /api/billing/transactions endpoint as described in issue #39

Changes

• Added GET /api/billing/transactions endpoint with authentication
• Supports query params: from, to, type (deposit|charge|withdrawal), limit, offset
• Extended UsageEventsRepository to fetch billing transactions (charges)
• Extended VaultRepository to fetch vault transactions (deposits/withdrawals)
• Merged transactions from both sources with consistent shape
• Added comprehensive tests for the endpoint
• Returns transactions with amount, date, type, tx_hash, and description

Testing

• Added unit tests in src/routes/billing.test.ts
• Tests cover authentication, validation, filtering, and pagination

Closes #39

[precious-akpan]

@greatest0fallt1me awaiting your review sir..

[greatest0fallt1me]

@precious-akpan Can you resolve the conflicts?