@mytec: feat: Phase 3.0 Architecture Refactor ✅

Major refactoring of RFCP backend:
- Modular propagation models (8 models)
- SharedMemoryManager for terrain data
- ProcessPoolExecutor parallel processing
- WebSocket progress streaming
- Building filtering pipeline (351k → 15k)
- 82 unit tests

Performance: Standard preset 38s → 5s (7.6x speedup)

Known issue: Detailed preset timeout (fix in 3.1.0)

backend/app/core/__init__.py

@@ -0,0 +1,6 @@
+"""
+Core business logic for RFCP.
+
+Existing modules: config.py, database.py
+New modules: engine.py, grid.py, calculator.py, result.py
+"""