fix: reuse pool_config for migrator tmp pool

[Taure]

Summary

• Migrator tmp pool now uses kura_app:pool_config() with database => "postgres" and pool_size => 1
• Inherits socket_options, so IPv6 (Fly.io) works for database creation
• Stops the tmp pool after use — previously leaked a connection to postgres DB for the app lifetime

Problem

The migrator manually built its pool config, missing socket_options. On Fly.io with IPv6-only Postgres, ensure_database failed with nxdomain because the tmp pool couldn't resolve the host.

Test plan

• 8 existing kura_app_tests pass
• Tested locally with asobi_arena via _checkouts
• Dialyzer clean

[github-actions]

🟢 Code Coverage — 93.5%

1927 of 2062 lines covered.

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🟡 ELP Lint — 1 warning

5 diagnostics found. See job logs for details.

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ℹ️ 11 OTP CVEs auto-ignored (already fixed in running version)

These CVEs are patched in the installed OTP version but NVD data
has not been updated to reflect this. They are excluded from the
scan via an auto-generated .trivyignore.

CVE	Details
CVE-2026-23943	Fixed in 28.4.1, running 28.4.1 — Pre-auth SSH DoS via unbounded zlib inflate
CVE-2026-23942	Fixed in 28.4.1, running 28.4.1 — SFTP root escape via component-agnostic prefix check in ssh_sftpd
CVE-2026-23941	Fixed in 28.4.1, running 28.4.1 — Request smuggling via first-wins Content-Length parsing in inets httpd
CVE-2026-21620	Fixed in 28.3.2, running 28.4.1 — TFTP Path Traversal
CVE-2016-1000107	Fixed in 28.0.4, running 28.4.1 — Httpd CGI Scripts Environment Variable Pollution AKA "httpoxy"
CVE-2025-58050	Fixed in 28.0.3, running 28.4.1 — Buffer Read Overflow on Regular Expressions with (*scs:) and (*ACCEPT)
CVE-2025-48038	Fixed in 28.0.3, running 28.4.1 — SSH Unverified File Handles can Cause Excessive Use of System Resources
CVE-2025-48039	Fixed in 28.0.3, running 28.4.1 — SSH Unverified Paths can Cause Excessive Use of System Resources
CVE-2025-48040	Fixed in 28.0.3, running 28.4.1 — SSH Malicious Key Exchange Messages may Lead to Excessive Resource Consumption
CVE-2025-48041	Fixed in 28.0.3, running 28.4.1 — SSH_FXP_OPENDIR may Lead to Exhaustion of File Handles
CVE-2025-4748	Fixed in 28.0.1, running 28.4.1 — Absolute Path in Zip Module