Rename Google Voice target to recipients

[Pigbibi]

Summary

• rename the Google Voice notification target from gateway to recipients
• keep the channel-specific config names while treating the destination as email-form recipients
• document that normal email recipients receive mail, while Google Voice addresses can also surface GV prompts

Tests

• PYTHONPATH=src pytest tests/test_google_voice_notifications.py tests/test_strategy_plugins.py
• PYTHONPATH=src ruff check .

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve legacy gateway key during recipients migration

This change makes StrategyPluginGoogleVoiceSettings.from_object read only crisis_alert_google_voice_recipients, so existing environments that still provide CRISIS_ALERT_GOOGLE_VOICE_GATEWAY will now parse zero recipients and be treated as misconfigured, causing every Google Voice alert to be skipped with missing_google_voice_config. Because this is a runtime config regression for already-deployed setups, the loader should accept both keys (at least during a deprecation window).

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