Add configurable --delay throttle for API requests

[Chisomnwa]

Summary

Adds a configurable --delay parameter to backfill_tags.py to throttle API requests during batch backfill, preventing rate limiting from Open Library.

What Changed

backfill_tags.py: Added --delay argument (float, default 0.5s) that controls the pause between API fetch requests during backfill. This lets callers adjust throttle rate per-run instead of editing source code.

Motivation

During production backfill planning, @jimchamp warned that unthrottled requests may be blocked by OL's rate limiting. This change adds a safe default delay (0.5s) while keeping it configurable per-run — use --delay 0.1 for faster runs or --delay 1.5 when being extra cautious.

Related

• Prerequisite for Phase 2c production backfill issue #46
• @mekarpeles previously suggested configurable delays over hardcoded values PR #40 review

Stakeholders

@mekarpeles @jimchamp

[jimchamp]

Noting that, as per our API usage guidelines, only 1 request per second can be made with no User-Agent. I don't believe that User-Agent can be set for requests made with the client (as per this open issue), so it may be good to set the default delay to one second.

[Chisomnwa]

Thanks @jimchamp, that makes sense. I'll update the defaults to 1 second delay and batch size 100.