@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/propagation/free_space.py

@@ -0,0 +1,43 @@
+"""
+Free Space Path Loss (FSPL) model.
+
+Used as baseline and for LOS conditions.
+FSPL = 20*log10(d) + 20*log10(f) + 32.45
+where d in km, f in MHz
+"""
+
+import math
+from app.propagation.base import PropagationModel, PropagationInput, PropagationOutput
+
+
+class FreeSpaceModel(PropagationModel):
+    """Free Space Path Loss — theoretical minimum propagation loss."""
+
+    @property
+    def name(self) -> str:
+        return "Free-Space"
+
+    @property
+    def frequency_range(self) -> tuple:
+        return (1, 100000)
+
+    @property
+    def distance_range(self) -> tuple:
+        return (1, 1000000)  # 1m to 1000km
+
+    def calculate(self, input: PropagationInput) -> PropagationOutput:
+        d_km = max(input.distance_m / 1000, 0.001)
+        f = input.frequency_mhz
+
+        L = 20 * math.log10(d_km) + 20 * math.log10(f) + 32.45
+
+        return PropagationOutput(
+            path_loss_db=L,
+            model_name=self.name,
+            is_los=True,
+            breakdown={
+                "distance_loss": 20 * math.log10(d_km),
+                "frequency_loss": 20 * math.log10(f),
+                "constant": 32.45,
+            },
+        )