feat: integrate external scheduler for prefetch task registration

Add SchedulerClient to communicate with external scheduler service for
registering volatile prefetch tasks discovered during HybridRAG searches.

- Add scheduler_client.py with full REST API for task CRUD operations
- Add scheduler_url config setting (default: http://scheduler:8090)
- Update consolidation service to use scheduler for prefetch registration
- Add scheduler health checks to startup/shutdown lifecycle

When HybridRAG classifies web content as prefetch-worthy, it now creates
scheduled tasks that periodically refresh the volatile cache via the
external scheduler service.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/config.py

@@ -128,6 +128,9 @@ class Settings(BaseSettings):
     system_settings_user: str = Field(default="settings", description="System settings database user")
     system_settings_password: str = Field(default="", description="System settings database password")
 
+    # Scheduler Service
+    scheduler_url: str = Field(default="http://scheduler:8090", description="Scheduler service URL")
+
     @property
     def qdrant_url(self) -> str:
         """Computed Qdrant URL."""