fix(#230): forward the per-job NIM API key for TXT/SRT/DOCX - #276

Problem

A NIM API key entered per job in the web UI only works for EPUB jobs. TXT, SRT and DOCX jobs silently drop it: with no NIM key in .env, the provider factory falls back to the empty .env value and the job fails even though a valid key was provided (#230).

Root cause

translate_file() forwards the per-job provider keys on the EPUB path only:

• the TXT/SRT adapter path builds llm_config without nim_api_key (src/core/adapters/translate_file.py:335-344)
• the DOCX path calls create_llm_provider() without it (src/core/adapters/translate_file.py:263-273)

The factory's nim branch (src/core/llm/factory.py:164-169) reads the kwarg before its env fallback, so the key never arrives and _require_key raises "NVIDIA NIM provider requires an API key". For TXT/SRT this surfaces mid-run as a failed unit (the job ends partial); for DOCX it raises at provider creation.

Fix

Thread nim_api_key at both call sites, matching the EPUB path. Two one-line kwargs; nothing else changes.

I also audited the sibling per-job keys (gemini, openai, openrouter, mistral, deepseek, poe) at both call sites, all are already forwarded; NIM was the only omission, at both sites.

How to reproduce (before the fix)

1. Start a job with provider nim, a per-job NIM API key filled in the web UI, and no NIM_API_KEY in .env.
2. TXT/SRT: every unit fails with "NVIDIA NIM provider requires an API key", the job ends partial, the output file is untranslated. DOCX: the job errors immediately at provider creation.
3. After this fix both formats translate and complete.

Testing

New tests/unit/test_translate_file_per_job_keys.py drives the real translate_file() entry point per format (txt, srt, docx) with llm_provider='nim', a per-job key and NIM_API_KEY absent; the real factory runs (its _require_key gate is the oracle), only the network boundary (provider.generate) is stubbed. All four tests fail on the old code (TXT/SRT: job ends partial; DOCX: the exact factory ValueError) and pass with the fix.

.venv/bin/python -m pytest tests/unit/test_translate_file_per_job_keys.py   # 4 passed
.venv/bin/python -m pytest tests/unit                                      # 1765 passed, 1 failed (pre-existing), 1 skipped

The single failure (test_path_validator.py::test_reject_path_separator_backslash) also fails on the clean base commit 0a26cec: it assumes \ is a path separator, which it is not on macOS/Linux. Unrelated to this change.

Changes

• src/core/adapters/translate_file.py: add nim_api_key to the DOCX create_llm_provider call and to the TXT/SRT llm_config
• tests/unit/test_translate_file_per_job_keys.py: regression tests (new file)

Closes #230