[codex] Add bounty ownership transfer endpoint

[caydyan]

Summary

• add a protected PATCH /gobounties/owner/transfer endpoint for moving unpaid workspace bounties from one owner pubkey to another
• require the requester to be the workspace owner, source owner, or hold workspace bounty-management permission
• require the target owner to exist and belong to the workspace before transfer
• add the database ownership update method and focused handler tests

Why

Addresses #1919. Core Lightning moved from a V1 account to a V2 account while active bounties remained owned by the V1 account. This provides a bounded migration path without exposing a broad unauthenticated owner rewrite.

Validation

• go test ./handlers -run TestTransferBountyOwnership -count=1
• go test ./db -run TestNonExistent -count=0
• go test ./... -run TestNonExistent -count=0
• git diff --check

[caydyan]

Current-head validation rerun on bd7f775:

• go test ./handlers -run TestTransferBountyOwnership -count=1 -v passed, including both ownership-transfer handler subtests.
• go test ./... -run ^$ passed as a compile/no-test package sweep.
• git diff --check origin/master...HEAD passed.

The focused handler test uses mocks, so it ran without the local Postgres test database.