feat(backend): B7 NDJSON streaming endpoint

[az-said]

Summary

• Adds POST /api/predict/stream returning application/x-ndjson for large-batch predictions.
• Event grammar per WAVE_2_8_DISPATCH §6.1: header (total) → N × row (or error) → footer (done).
• Per-peptide isolation: a single bad sequence emits an error event but does not abort the stream.
• anyio.to_thread.run_sync runs each predict in a worker thread; await anyio.sleep(0) between peptides gives the loop a chance to detect a client disconnect.
• X-Accel-Buffering: no header so nginx in front of the Hetzner prod box does not batch the response into one chunk.
• Existing POST /api/predict and POST /api/predict/batch routes are unchanged.

Test plan

• `pytest backend/tests/test_predict_stream.py -v` — 4 cases all green
  • basic envelope (header → 3 rows in order → footer)
  • error event isolates a bad peptide without killing the stream
  • empty peptide list → 422
  • missing required field → 422
• Backend regression: 623 of pre-existing 624 tests still pass (the 1 failure in `test_esm2_embedder` is an unrelated env-var precedence quirk).
• Manual: `curl -N -X POST http://localhost:8000/api/predict/stream -H "Content-Type: application/json" -d '{"peptides":[{"id":"u1","sequence":"GVGDLIRKAVSVIKNIV"}],"config":{}}' | head -20`

Out of scope (per dispatch)

• No changes to backend/schemas/api_models.py response shapes.
• No changes to process_upload_dataframe internals.

Closes B7 from `docs/internal/WAVE_2_8_DISPATCH_2026_06_18.md` §3.

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 834d90a and 01ae3ae.

📒 Files selected for processing (2)

• backend/api/routes/predict.py
• backend/tests/test_predict_stream.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/backend-b7-ndjson-stream

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