Close the remaining places where an extension point was hardcoded, and
expose a stable public import surface for every app.
- files: register_storage_backend(...) / STORAGE_BACKENDS[name]["CLASS"];
unknown backends now fail loudly instead of silently using local
- files: PipelineStepRegistry + @pipeline_step; PIPELINE_EXECUTOR setting
- billing: INVOICE_PDF_BUILDER setting
- security: TWO_FACTOR_SERVICE / TWO_FACTOR_RECOVERY_SERVICE settings
- all app packages expose lazy public exports (PEP 562); infrasynth.shared
re-exports its primitives eagerly
- README documents the per-module extension-point table
- tests/test_extensibility.py pins each hook plus the public surface
