Add Google Translate engine for paper PDF jobs

[znsoftm]

Hub admins can now pick Google Translate for paper PDF translation in Settings (/settings, papers translate section). The choice persists in $PAPERS_DIR/translate-config.json and applies to new BabelDOC jobs without a redeploy.

Why a shim (not --google)

Current BabelDOC (0.6.x on the papers stack) only exposes --openai. Older BabelDOC/pdf2zh builds had --google; this one does not. Inventing a fake engine that still called Hub would not meet the goal.

This PR keeps BabelDOC on --openai and points it at a local OpenAI-compatible shim that actually calls Google:

1. Google Cloud Translation API v2 when an optional google_api_key is saved
2. Otherwise the public Google Translate web endpoints (translate.googleapis.com/translate_a/single, fallback translate.google.com/m) — the same family pdf2zh-next uses

The Python worker (agent-papers/google_translate.py + translate_worker.py) is the primary path. The Go BabelDOC fallback starts the same kind of shim.

What stays on Hub LLM

engine is PDF-only. Abstract ZH, paper review, and Hugging Face daily trend still use the existing Hub / OpenAI-compatible snapshot (base_url / api_key / model). Those need chat completions; Google is a machine-translation path, not a chat model.

Settings / credentials

• Radio: Hub / OpenAI-compatible LLM vs Google 翻译 / Google Translate
• Active-engine badge + short hint that Google is faster but quality/layout may differ
• Optional masked Google Cloud Translation API key (google_api_key); empty does not wipe; clear_google_api_key deletes
• No secrets committed (translate-config.json remains gitignored)
• Same Hub global-admin auth as existing settings
• zh/en i18n for the new labels
• Test button pings Google when Google is selected (Cloud if a key is present, else the web endpoint)

Tests

• Config persistence, engine validation, Google-without-Hub-key job routing, Hub-still-requires-key
• Google Cloud vs web client routing, placeholder protection, OpenAI shim extracts BabelDOC Input: payload only
• Settings HTML includes the engine control
• Python unit tests for extract / routing / shim

Ops notes

• Google selected + Cloud key: uses paid Cloud Translation; auth failures fail clearly (no silent Hub fallback)
• Google selected + no key: uses the public web path (works without a paid key; may be rate-limited)
• Hub selected: today’s BabelDOC --openai + translate-config behavior unchanged