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>

deploy/.env.example

@@ -57,16 +57,34 @@ SMTP_USERNAME=
 SMTP_PASSWORD=
 SMTP_FROM=
 
-# --- Model providers (AI assistant + embeddings; both optional, default off) ---
-# LLM: 'null' disables AI features; 'anthropic' uses the Claude API.
-MODEL_PROVIDER=null
-ANTHROPIC_API_KEY=
-LLM_MODEL=claude-opus-4-8
+# --- Model providers (AI assistant + embeddings) -----------------------------
+# Configure as many as you like — each turns on when its key is set. The
+# default_* vars pick which one is used by default; the app can also select any
+# configured provider by name. LLM and embeddings are independent (Anthropic has
+# no embeddings endpoint). Leave the defaults 'null' to keep AI off.
+DEFAULT_LLM_PROVIDER=null          # null | anthropic | openai | xai | ollama
+DEFAULT_EMBEDDING_PROVIDER=null    # null | openai | ollama
 LLM_MAX_TOKENS=4096
-# Embeddings are separate (Anthropic has no embeddings endpoint). 'null' for now.
-EMBEDDING_PROVIDER=null
+EMBEDDING_DIMENSIONS=1536          # must match the embedding model + pgvector column
 
-# --- Model providers — wired in Phase 4 (AI assistant). BYO key. ---
-# ANTHROPIC_API_KEY=
-# OPENAI_API_KEY=
+# Anthropic (LLM)
+ANTHROPIC_API_KEY=
+ANTHROPIC_MODEL=claude-opus-4-8
+
+# OpenAI (LLM + embeddings)
+OPENAI_API_KEY=
+OPENAI_BASE_URL=https://api.openai.com/v1
+OPENAI_MODEL=gpt-4o
+OPENAI_EMBEDDING_MODEL=text-embedding-3-small
+
+# xAI / Grok — OpenAI-compatible (LLM)
+XAI_API_KEY=
+XAI_BASE_URL=https://api.x.ai/v1
+XAI_MODEL=grok-2-latest            # set to your account's current Grok model
+
+# Ollama — local, OpenAI-compatible, no key (LLM + embeddings)
+OLLAMA_ENABLED=false
+OLLAMA_BASE_URL=http://localhost:11434/v1
+OLLAMA_MODEL=llama3.1
+OLLAMA_EMBEDDING_MODEL=nomic-embed-text
 # XAI_API_KEY=