feat: make Ollama/gemma4 the primary backend with Claude as fallback

Rolls back the claudification backend preference: PREFER_CLOUD_BACKEND now
defaults to false, resolve_backend() picks Ollama first and uses Claude when
explicitly preferred or when the new Ollama startup health check fails. The
Steward retries mid-request failures on the other backend in both directions.

Also hardens the fallback itself: Anthropic SDK imports are lazy so a broken
anthropic package degrades to Ollama-only instead of crashing at import time
(root cause of the production outage since April), anthropic is pinned to a
pydantic-ai-1.27-compatible range, ANTHROPIC_MODEL defaults to claude-sonnet-5
(sonnet-4-20250514 retired 2026-06-15), sampling parameters are stripped from
Claude calls (Sonnet 5 rejects them), and the Steward timeout is configurable
(STEWARD_TIMEOUT, default 60s) since gemma4 needs ~35s warm for analysis.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/core/config.py

@@ -64,21 +64,21 @@ class Config(BaseSettings):
     API_PORT: int = Field(default=8000, description="API port")
     API_PREFIX: str = Field(default="/v1", description="API route prefix")
     
-    # Anthropic Configuration (Claude - preferred backend)
+    # Anthropic Configuration (Claude - cloud fallback)
     ANTHROPIC_API_KEY: str | None = Field(
         default=None,
-        description="Anthropic API key for Claude access"
+        description="Anthropic API key for the Claude fallback backend"
     )
     ANTHROPIC_MODEL: str = Field(
-        default="claude-sonnet-4-20250514",
-        description="Claude model to use"
+        default="claude-sonnet-5",
+        description="Claude model for the fallback backend"
     )
     PREFER_CLOUD_BACKEND: bool = Field(
-        default=True,
-        description="Prefer Claude over Ollama when available"
+        default=False,
+        description="Prefer Claude over Ollama (default: local-first)"
     )
 
-    # Ollama Configuration (local fallback)
+    # Ollama Configuration (local - primary backend)
     OLLAMA_HOST: HttpUrl = Field(
         default="http://localhost:11434",
         description="Ollama server URL"
@@ -91,6 +91,10 @@ class Config(BaseSettings):
         default=120,
         description="Ollama request timeout in seconds"
     )
+    STEWARD_TIMEOUT: int = Field(
+        default=60,
+        description="Steward analysis timeout in seconds (gemma4 needs ~35s warm)"
+    )
     STREAM_TIMEOUT: int = Field(
         default=20,
         description="Timeout for each streaming turn in seconds"