feat(subscription): add external custom configs

[mustfqyl]

Summary

Adds support for appending externally managed share links to generated subscriptions.

• Adds an External Configurations field to subscription settings.
• Supports multiple external links, one per line.
• Appends external links to plain-text and Base64 subscriptions.
• Supports subscription template variables such as {USERNAME}.
• Preserves backward compatibility with the EXTERNAL_CONFIG environment variable.
• Adds English, Persian, Russian, and Chinese translations.

Closes #683

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

• uv run pytest -q tests/test_subscription_external_config.py — 3 passed.
• Ruff checks passed for the changed backend and test files.
• TypeScript type checking completed successfully.
• JSON validation passed for all four modified locale files.
• A full dashboard build could not be completed in the validation environment because the installed @pasarguard/core-kit package did not contain its expected dist output.

No database migration is required because this setting is stored in the existing JSON settings structure.

Screenshots

Not available. The change adds a multiline External Configurations field to the existing subscription settings form.

Notes for reviewers

External links are intentionally appended only to links and links_base64 outputs. Raw share links cannot safely be appended directly to structured Xray, Clash, or Sing-box JSON/YAML outputs.

Existing installations using the EXTERNAL_CONFIG environment variable remain supported.

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