fix: support backfill script in api image

[suguanYang]

Summary

• fix backfill_map_unit_indexes.py for the production API image layout
• resolve packages/shared-python from both the source checkout and /app runtime image
• add contract coverage for both layouts
• unblock the post-deploy operation documented by PR perf: persist map-nav scoring index for large corpora #344

Verification

• PYTHONPATH=apps/api:packages/shared-python /home/suguan/github.com/ontosAI/knowhere/.venv/bin/pytest -q apps/api/tests/contract/test_backfill_map_unit_indexes_contract.py
• Ruff format and check passed for changed files.
• Production read-only inventory task using v1.0.32-prod reproduced the container-path IndexError; no backfill writes were performed.

Deployment Notes

• no database migration or API contract changes
• deploy this fix together with the existing PR perf: persist map-nav scoring index for large corpora #344 release line, then rerun the read-only inventory before canary/full backfill
• the prior temporary task definition knowhere-api-prod-backfill:1 can be deregistered after the operation plan is finalized; it was not used for writes

Checklist

• Tests were added or updated when behavior changed
• Public docs, examples, or OpenAPI contracts were updated when needed
• Database migrations are idempotent and safe to deploy (no migration changed)
• Logs, errors, and validation paths avoid leaking secrets or user data
• The pull request description explains any breaking or user-visible change