feat(hosts): add Xray sockopt domain strategy

[TaraRostami]

Summary

Adds a per-host xray_sockopt_domain_strategy option to control streamSettings.sockopt.domainStrategy on generated Xray JSON proxy outbounds.

The default is AsIs and remains implicit for backward compatibility, so existing hosts keep the same generated Xray configuration unless a different strategy is selected.

This is useful when a generated Xray client configuration needs Xray's built-in DNS resolver to resolve the outbound server hostname, while keeping the hostname itself in the Host configuration and other subscription formats.

The setting merges with existing Fragment/Noise dialerProxy configuration and is applied to generated VMess/VLESS/Trojan/Hysteria outbounds, Shadowsocks, and XHTTP download-side stream settings.

WireGuard keeps its existing protocol-specific settings.domainStrategy, and non-Xray subscription formats such as share links remain unchanged.

Important

The migration currently uses 7c4bd5128e62 as its down_revision, which was the
Alembic head of dev when this PR was prepared. If new migrations are merged into
dev before this PR, this migration should be rebased onto the new head to avoid
creating multiple Alembic heads.

Type of change

• Bug fix
• New feature
• Breaking change
• Refactor / cleanup
• Documentation
• Tests / CI

Checklist

• I tested the change locally or explained why it cannot be tested.
• I added or updated tests for behavior changes.
• I updated documentation, translations, or examples if needed.
• I checked database migrations when models or schema changed.
• I did not include secrets, tokens, private keys, or unrelated changes.

Testing

Tested locally on Windows with a development SQLite database.

Backend / migration checks:

uv run alembic upgrade head
uv run alembic current
make check-migrations
make check

Feature-specific tests

uv run pytest tests/test_xray_sockopt_domain_strategy.py -v
uv run pytest tests/api/test_user.py -k xray_subscription_applies_host_sockopt_domain_strategy -v

The dedicated regression tests cover:

• Hysteria streamSettings.sockopt.domainStrategy
• Shadowsocks' independent outbound builder
• WireGuard retaining its protocol-specific settings.domainStrategy
• API → database → subscription propagation
• AsIs remaining implicit in generated Xray JSON
• merging domainStrategy with Fragment/Noise dialerProxy

The panel was also run locally and the feature was manually tested through the Host UI and generated Xray subscription output.

Screenshots

Notes for reviewers

• AsIs is the default and is intentionally omitted from generated Xray JSON to preserve existing behavior.
• Non-AsIs values are emitted as streamSettings.sockopt.domainStrategy.
• Existing Fragment/Noise dialerProxy settings are merged rather than overwritten.
• XHTTP download-side stream settings use the selected strategy in Xray JSON output.
• WireGuard is intentionally excluded because it already uses the protocol-specific settings.domainStrategy.
• Share-link and other non-Xray subscription formats are unchanged.
• The database migration adds the new host column with AsIs as a non-null server default.

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