[codex] Remove migrate bounties endpoint

[cybercraftsolutionsllc]

Summary

• remove the leftover public /migrate_bounties route and MigrateBounties handler
• remove the obsolete migration handler tests
• remove the endpoint from generated Swagger docs

Validation

• go test handlers/bounties.go -run '^$' -count=1
• go test ./routes -run '^$' -count=1
• rg -n "migrate_bounties|MigrateBounties" handlers routes docs -g "*.go" -g "*.json" -g "*.yaml" returned no references
• git diff --check

Notes

• go test ./handlers -run TestGetWantedsHeader -count=1 is blocked by existing stale mock compile errors around GetBountyByUnlockCode.
• go test handlers/bounties.go handlers/bounties_test.go -run '^TestGetWantedsHeader$' -count=1 is blocked by the local Postgres test DB not running on localhost:5532.
• go test ./routes -run '^TestNewRouter$' -count=1 is blocked by existing test setup/runtime behavior around nil DB feature-flag middleware and xgo warnings; package compile with no tests passes.

Refs #597