Fix #215: pluggable LLM + embedding provider abstraction

Adds the vendor-agnostic seam the AI assistant + match-ranking plug into:
- LLMProvider / EmbeddingProvider ABCs (base.py). LLM and embeddings are
  SEPARATE abstractions — Anthropic has no embeddings endpoint, so each is
  configured independently and either can be off.
- NullLLMProvider / NullEmbeddingProvider — the default; fail loud with a clear
  "not configured" error so AI-off deployments don't silently no-op.
- AnthropicLLMProvider — first concrete LLM impl, via the official anthropic SDK
  (default model claude-opus-4-8). A local provider (e.g. Ollama) would be
  another subclass of the same interface.
- Factory in deps.py (get_llm_provider / get_embedding_provider) selects by
  env (MODEL_PROVIDER / EMBEDDING_PROVIDER); documented in .env.example.

Providers are read-only text/vector producers — they never touch the DB, so the
"AI never writes autonomously" invariant (CLAUDE.md #1) holds; writes will go
through ChangeProposal (#214).

Tests: provider selection (null default, anthropic when keyed, fallback without
key) + null providers raise. 81 passed.

Closes #215

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

backend/app/api/deps.py

@@ -10,6 +10,8 @@ from app.core.db import get_session
 from app.integrations.mailer.base import Mailer
 from app.integrations.mailer.console import ConsoleMailer
 from app.integrations.mailer.smtp import SMTPMailer
+from app.integrations.models.base import EmbeddingProvider, LLMProvider
+from app.integrations.models.null import NullEmbeddingProvider, NullLLMProvider
 from app.integrations.objectstore.base import ObjectStore
 from app.integrations.objectstore.s3 import S3ObjectStore
 from app.models.user import User
@@ -67,3 +69,28 @@ def get_objectstore() -> ObjectStore:
 
 
 ObjectStoreDep = Annotated[ObjectStore, Depends(get_objectstore)]
+
+
+def get_llm_provider() -> LLMProvider:
+    settings = get_settings()
+    if settings.model_provider == "anthropic" and settings.anthropic_api_key:
+        from app.integrations.models.anthropic_provider import AnthropicLLMProvider
+
+        return AnthropicLLMProvider(
+            api_key=settings.anthropic_api_key,
+            model=settings.llm_model,
+            max_tokens=settings.llm_max_tokens,
+        )
+    return NullLLMProvider()
+
+
+LLMProviderDep = Annotated[LLMProvider, Depends(get_llm_provider)]
+
+
+def get_embedding_provider() -> EmbeddingProvider:
+    # Only the null provider exists today; concrete embedders (Ollama/Voyage)
+    # implement the same interface and are selected here by settings.embedding_provider.
+    return NullEmbeddingProvider()
+
+
+EmbeddingProviderDep = Annotated[EmbeddingProvider, Depends(get_embedding_provider)]