feat: cli to reindex elastic guests index

[Dao-Ho]

Adds a reindex-guests CLI command that backfills OpenSearch from Postgres.

• AllGuestDocuments on GuestsRepository — cursor-paginated iter.Seq2 generator, yields one doc at a time without loading the full table into memory
• BulkIndexGuests on OpenSearchGuestsRepository — sends a batch as a single NDJSON BulkRequest, surfaces per-document errors from the response
• CLI accumulates docs into 100-doc batches and flushes via bulk index, so DB reads and OpenSearch writes are both paged
• Documents the pattern in CLAUDE.md

[codecov]

Codecov Report

❌ Patch coverage is 0% with 133 lines in your changes missing coverage. Please review.
✅ Project coverage is 18.20%. Comparing base (8b18b59) to head (79bede8).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
backend/internal/repository/guests.go	0.00%	61 Missing ⚠️
backend/internal/repository/guests_opensearch.go	0.00%	40 Missing ⚠️
backend/cmd/cli/guests.go	0.00%	32 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #259      +/-   ##
==========================================
- Coverage   19.82%   18.20%   -1.63%     
==========================================
  Files          40       41       +1     
  Lines        1493     1626     +133     
==========================================
  Hits          296      296              
- Misses       1193     1326     +133     
  Partials        4        4              

Flag	Coverage Δ
backend	18.20% <0.00%> (-1.63%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
backend/cmd/cli/main.go	0.00% <ø> (ø)
backend/cmd/cli/guests.go	0.00% <0.00%> (ø)
backend/internal/repository/guests_opensearch.go	0.00% <0.00%> (ø)
backend/internal/repository/guests.go	0.00% <0.00%> (ø)

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[eric-kitagawa]

LGTM