Smartstack Scaffolding

[timbeccue]

Add the infrastructure for smartstack observations. This includes the stacking core/data model and the
pipeline integration to receive and process subframes.

• Add StackFrame DB model and helper functions to track frames during smartstack observations
• Add stacking worker (Redis-based per-camera notifications, stack completion check, timeout detection)
• Add stacking supervisor (runs a separate stack worker per camera)
• Add SubframeListener (similar to banzai-listener)
• Add process_subframe worker (similar to banzai-worker, but also sends reduced subframes to the stack worker)
• Add banzai_subframe_worker entry point
• Modify run_pipeline_stages to return the reduced image filepath, for passing to the stack DB/worker
• Unit tests and integration test scaffolding with Docker Compose

These changes add everything needed for smartstacking except the stacking algorithm itself.

Note: stage_utils.py:run_pipeline_stages previously had no return value and now returns the processed
images. This shouldn't affect existing callers but is worth flagging since it modifies a core banzai
function.