feat(hosts): add per-host client template overrides

[x0sina]

Summary

This PR adds per-host client template overrides so each host can choose custom client templates by output type while preserving existing default fallback behavior.

It introduces a single host-side client_template_ids field to store optional overrides for:

• Clash / Clash Meta subscription template
• Xray subscription template
• Sing-box subscription template
• HTTP User-Agent template
• gRPC User-Agent template

When an override is not set, invalid, deleted, or not consistently applicable for the rendered request, subscription generation falls back to the existing default template behavior.

Backend Changes

• Added client_template_ids JSON storage on hosts with migration support.
• Exposed the override structure in host API models and cached subscription host data.
• Added backend validation to ensure each configured host override points to a client template of the correct type.
• Extended client-template cache/catalog loading to support lookup by template id as well as by default template type.
• Updated subscription rendering to resolve host-level template overrides per request and fall back safely to defaults.

Dashboard Changes

• Added host form support for per-type client template overrides.
• Updated host create/edit/duplicate/reorder flows to preserve client_template_ids.
• Added a compact host modal section under Routing for managing overrides.
• Excluded default templates from override dropdowns to avoid showing the same default choice twice.
• Added translations for the new host template UI in English, Persian, Russian, and Chinese.

Notes

• Storage uses a single JSON host column instead of one database column per template type.
• I did not complete a full automated test pass in this environment.

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