docs(config): clarify environment and backfill inputs

[f0rr0]

Summary

• reconcile .env.example with the typed environment schema and current runtime/script consumers
• distinguish cron, manual-backfill, and webhook secrets by their actual responsibilities
• document intentional omission of platform-provided variables and compatibility aliases
• keep the canonical key set identical to the local .env.local template
• clarify that the backfill start and end dates are both included and that the entire range may contain at most 366 calendar days

Validation

• .env.example and .env.local canonical key sets match
• actionlint .github/workflows/github-activity-backfill.yml
• bun run format:check
• git diff --check

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Project	Deployment	Actions	Updated (UTC)
f0rr0-github-io	Ready	Preview	Aug 29, 2026 5:19pm