fix: resolve unsafe nightly backfill window from invalid env values

[RealDiligent]

Root cause

RepoBackfillScheduleService parsed NIGHTLY_BACKFILL_DAYS with raw Number(...) and used it directly for nightly backfill jobs. Invalid/non-finite or out-of-range values could cause excessive or malformed backfill windows and unnecessary load across GitHub API, queue workers, and DB.

Fix approach

• Added bounded parsing for NIGHTLY_BACKFILL_DAYS with finite checks and integer normalization.
• Enforced safe range (1..365) with fallback to DEFAULT_BACKFILL_DAYS.
• Added startup warning when fallback is used.
• Used validated value consistently for logs and queued nightly backfill jobs.

Impact

• Prevents unsafe nightly backfill windows caused by env misconfiguration.
• Preserves existing behavior for valid configurations.

Risk / tradeoffs

• Invalid/out-of-range env values now fall back to defaults instead of being accepted.

Made with Cursor

[RealDiligent]

CI workflows are in \�ction_required\ because this PR comes from a fork and needs maintainer approval to run.\n\nLocal validation is already green:\n- npm run lint\n- npm run build\n- npm test\n\nAfter workflow approval, checks should execute normally.