[P1] Build the transaction timeline from all evidence tables

[samithreddychinni]

Problem

internal/adminapi/transactions.go:158 builds events from ledger only. Polls and the latest webhook are returned separately, but the dashboard timeline in dashboard/src/pages/Transactions/TimelineDrawer.jsx expects event types such as hold_created, webhook_received, poll_completed, and callback_delivered in the event stream.

Right now the timeline can miss the evidence that explains a transaction. It shows state transitions, but not the webhook and poll history that caused them.

Scope

• Change buildTimeline to merge relevant rows from holds, webhooks, verification_polls, ledger, and callback delivery evidence.
• Include event types for hold creation, webhook receipt, rejected webhook if tied to a transaction, poll completion, poll failure, state transition, callback delivered, and callback failed.
• Sort the merged events by timestamp.
• Include useful data fields: gateway status, gateway amount, poll attempt, error text, webhook event type, callback HTTP status, and transition from and to values.
• Keep polls if the UI still needs the tabular poll view, but do not make the main timeline ledger-only.
• Avoid leaking secrets or raw signed payload fields in timeline summaries.

Acceptance criteria

• A transaction with a webhook and polls shows those records in events in timestamp order.
• A transaction with failed polls shows the poll errors in events.
• A delivered callback appears in events from the delivery evidence.
• Existing timeline drawer behavior works without mock-only event types.
• Admin API tests cover a merged timeline with at least one webhook, two polls, one transition, and one callback event.