feat(sre): enforce typed controlled automation

apps/api/src/core/config.py

@@ -486,6 +486,20 @@ class Settings(BaseSettings):
         le=3600,
         description="Gemini failed generation work-item cooldown seconds",
     )
+    GEMINI_EXECUTION_MODE: str = Field(
+        default="shadow",
+        pattern="^(shadow|canary|production)$",
+        description=(
+            "Gemini route mode: shadow performs no API call; canary uses a "
+            "deterministic run bucket; production enables the final fallback"
+        ),
+    )
+    GEMINI_CANARY_PERCENT: int = Field(
+        default=0,
+        ge=0,
+        le=100,
+        description="Deterministic Gemini canary percentage when mode=canary",
+    )
     CLAUDE_MODEL: str = Field(
         default="claude-sonnet-5",
         min_length=1,