Use bulk issue-add endpoint for adding issues to Wave Programs

[efstajas]

Frontend half of drips-network/wave#735 — requires drips-network/wave#739 to be deployed first.

Change

The "add issues to Wave Program" flow now sends one POST /wave-programs/:id/issues/bulk request per chunk of 50 issues (the server's cap), sequentially, instead of one request per issue. Each chunk is a single backend transaction with a single org-lock acquisition, so bulk adds no longer pile up advisory-lock waiters server-side (the cause of today's statement-timeout 503 storms; #1945 was the stopgap).

• New addIssuesToWaveProgram util + response schemas in $lib/utils/wave.
• Per-issue failures come from the bulk response's results and feed the same partial-failure error screen as before; whole-chunk failures (network/5xx) mark just that chunk's issues as failed while other chunks proceed.
• Successful adds are refreshed and wave:issues invalidated exactly as before, including on partial failure.
• mapWithConcurrency from throttle bulk wave issue adds #1945 is left in place (generic, tested util) but the add flow no longer uses it.

Testing

• npm run check, eslint, prettier clean; unit tests pass (106).
• Flow behavior on partial failure (some issues closed/over budget) unchanged: added issues refresh, failed ones listed per-issue on the error screen.

Deploy order

Backend drips-network/wave#739 must ship first — this flow 404s against an API without the bulk route.

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