feat: add ADMIN_DATABASE_URL setting with fallback to DATABASE_URL

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/core/config.py

@@ -28,6 +28,20 @@ class Settings(BaseSettings):
         """Get sync URL by removing asyncpg prefix from DATABASE_URL."""
         return self.DATABASE_URL.replace("postgresql+asyncpg://", "postgresql://", 1)
 
+    # Admin database — resolutionflow_admin role, BYPASSRLS.
+    # Used by /admin/* endpoints. Defaults to DATABASE_URL for local dev.
+    ADMIN_DATABASE_URL: str = ""
+
+    @field_validator("ADMIN_DATABASE_URL", mode="before")
+    @classmethod
+    def default_admin_database_url(cls, v: str, info) -> str:
+        """Fall back to DATABASE_URL if ADMIN_DATABASE_URL is not set."""
+        if not v:
+            return info.data.get("DATABASE_URL", "")
+        if v.startswith("postgresql://"):
+            return v.replace("postgresql://", "postgresql+asyncpg://", 1)
+        return v
+
     # JWT Settings
     SECRET_KEY: str = _DEFAULT_SECRET_KEY