Vision backends: any OpenAI-compatible endpoint, including local models

The Anthropic key was the last hard gate for other users. extract's
VisionFn seam gains a second factory speaking the chat-completions
format — OpenAI, OpenRouter, or a local runtime (Ollama, LM Studio,
llama.cpp, vLLM) via config.toml: vision_provider, vision_base_url,
and vision_key_env ("" = keyless local endpoint, no Authorization
header sent). Anthropic stays the default. load_config rejects unknown
providers loudly, the pipeline page's credentials warning follows the
configured provider (a keyless local endpoint warns about nothing),
and config.toml + README document the local-model trade honestly:
weaker spine reading means a longer proofread pass, which the shaky-
read workflow absorbs.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016jXZFSTZQKzAC8fqpWSz9g

README.md

@@ -70,7 +70,7 @@ Every stage is idempotent and resumable: kill it mid-run, restart, lose nothing.
 ## Requirements
 
 - macOS or Linux, Python 3.12+, [uv](https://docs.astral.sh/uv/)
-- An [Anthropic API key](https://console.anthropic.com/) (vision extraction)
+- A vision model for extraction — an [Anthropic API key](https://console.anthropic.com/) by default, or any OpenAI-compatible endpoint via `config.toml` (`vision_provider = "openai-compatible"` + `vision_base_url`): OpenAI, OpenRouter, or a free local runtime like [Ollama](https://ollama.com/) with a vision-capable model. Local models read spines noticeably worse than frontier ones — expect a longer proofread pass on the Titles page, not a broken pipeline.
 - A BoardGameGeek account **and a registered BGG application** — as of BGG's [2025 API policy](https://boardgamegeek.com/using_the_xml_api), the XML API requires a Bearer token from a registered app. Register a free non-commercial application at [boardgamegeek.com/applications](https://boardgamegeek.com/applications) (approval can take a week or more, so **apply on day one**), then create a token. Each user needs their own; tokens must not be shared.
 
 ## Quick start