[P1] Start verification polls when a hold is created

[samithreddychinni]

Problem

A verification poll is only queued when a webhook is persisted in internal/webhook/handler.go:159. internal/hold/store.go:35 creates the hold but does not create any verification work.

If the webhook never arrives, the transaction can sit in PENDING until the TTL scanner runs a final check. That is too late. Missing webhooks are normal gateway behavior and should be handled by the poll schedule.

Scope

• Queue the first verification_polls row when a hold is created.
• Make the hold insert and initial poll insert atomic, or make retry behavior explicit and tested.
• Keep webhook-triggered polling useful, but prevent duplicate attempt 1 rows when both hold creation and webhook receipt happen.
• Add a uniqueness guard for poll attempts, for example one row per txn_id and attempt_number, if that is the chosen approach.
• Use the existing lag schedule for later attempts. Do not invent a second schedule in the hold package.
• Decide whether the first poll is immediate or scheduled after the first configured delay, then test that decision.

Acceptance criteria

• Creating a hold creates one pending verification poll without waiting for a webhook.
• Receiving a webhook for that hold does not create a duplicate attempt 1 poll.
• A hold with no webhook gets at least one gateway status check before TTL expiry.
• Duplicate identical hold create requests do not create extra polls.
• Tests cover create only, webhook only, create then webhook, and duplicate create cases.