feat: send Discord notifications for waitlist and new Google signups

[anxi01]

Summary

• send a best-effort Discord webhook after a waitlist signup is saved
• format waitlist Discord embeds with the user, submitted video link, signup time, and YouTube thumbnail preview
• send a separate best-effort Discord webhook when a user signs in with Google for the first time

Details

• adds DISCORD_WAITLIST_WEBHOOK_URL for waitlist notifications
• adds DISCORD_NEW_USER_WEBHOOK_URL for first-time Google signup notifications
• keeps both webhook paths non-blocking so API success does not depend on Discord delivery
• documents the new env vars in .env.example and README.md

Testing

• python3 -m pytest -q

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Treat naive created_at as UTC before converting timestamp

entry.created_at.astimezone(UTC) produces incorrect epoch values when created_at is naive (common with SQLite and some DB/driver timezone settings), because Python assumes the host local timezone for naive datetimes. In non-UTC deployments this shifts the displayed Discord "Signed Up" time by the server offset, so notifications show wrong signup times even though the stored value represents UTC.

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