Model providers: OpenAI/xAI/Ollama + run several at once (registry)

Extends the #215 abstraction:
- OpenAICompatibleLLMProvider / OpenAICompatibleEmbeddingProvider — one impl (via
  the official openai SDK) covers OpenAI, xAI (api.x.ai/v1), Ollama
  (…:11434/v1), OpenRouter, etc.; they differ only by base_url, key, and model.
- Registry factory: build_llm_providers() / build_embedding_providers() return
  every provider whose credentials are configured, so you can run several
  concurrently. get_llm_provider(name)/get_embedding_provider(name) select by
  name, falling back to default_*_provider, then Null.
- Per-provider env config (ANTHROPIC_*, OPENAI_*, XAI_*, OLLAMA_*) +
  DEFAULT_LLM_PROVIDER / DEFAULT_EMBEDDING_PROVIDER; documented in .env.example.
  Defaults keep AI off (empty registry).

Embeddings now have real backends (OpenAI/Ollama), still separate from the LLM
since Anthropic offers no embeddings endpoint. Tests cover multi-provider
selection, default resolution, disabled-without-credentials, and null fail-loud.
Full suite 87 passed.

Relates to #215.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

backend/uv.lock

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+[[package]]
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+source = { registry = "https://pypi.org/simple" }
+dependencies = [
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+    { name = "distro" },
+    { name = "httpx" },
+    { name = "jiter" },
+    { name = "pydantic" },
+    { name = "sniffio" },
+    { name = "tqdm" },
+    { name = "typing-extensions" },
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+wheels = [
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 version = "26.2"
@@ -578,6 +597,7 @@ dependencies = [
     { name = "asyncpg" },
     { name = "boto3" },
     { name = "fastapi" },
+    { name = "openai" },
     { name = "pydantic" },
     { name = "pydantic-settings" },
     { name = "python-multipart" },
@@ -601,6 +621,7 @@ requires-dist = [
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     { name = "boto3", specifier = ">=1.35" },
     { name = "fastapi", specifier = ">=0.115" },
+    { name = "openai", specifier = ">=2.41.0" },
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     { name = "pydantic-settings", specifier = ">=2.5" },
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+[[package]]
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+source = { registry = "https://pypi.org/simple" }
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 version = "4.15.0"