@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/cost231_hata.py

@@ -0,0 +1,62 @@
+"""
+COST-231 Hata model (extension of Okumura-Hata).
+
+Valid for:
+- Frequency: 1500-2000 MHz
+- Distance: 1-20 km
+
+Better for LTE bands than original Okumura-Hata.
+"""
+
+import math
+from app.propagation.base import PropagationModel, PropagationInput, PropagationOutput
+
+
+class Cost231HataModel(PropagationModel):
+
+    @property
+    def name(self) -> str:
+        return "COST-231-Hata"
+
+    @property
+    def frequency_range(self) -> tuple:
+        return (1500, 2000)
+
+    @property
+    def distance_range(self) -> tuple:
+        return (100, 20000)
+
+    def calculate(self, input: PropagationInput) -> PropagationOutput:
+        f = input.frequency_mhz
+        d = max(input.distance_m / 1000, 0.1)
+        hb = max(input.tx_height_m, 1.0)
+        hm = max(input.rx_height_m, 1.0)
+
+        # Mobile antenna correction (medium city)
+        a_hm = (1.1 * math.log10(f) - 0.7) * hm - (1.56 * math.log10(f) - 0.8)
+
+        # Metropolitan center correction
+        C_m = 3 if input.environment == "urban" else 0
+
+        L = (
+            46.3
+            + 33.9 * math.log10(f)
+            - 13.82 * math.log10(hb)
+            - a_hm
+            + (44.9 - 6.55 * math.log10(hb)) * math.log10(d)
+            + C_m
+        )
+
+        return PropagationOutput(
+            path_loss_db=L,
+            model_name=self.name,
+            is_los=False,
+            breakdown={
+                "base_loss": 46.3,
+                "frequency_term": 33.9 * math.log10(f),
+                "height_gain": -13.82 * math.log10(hb),
+                "mobile_correction": -a_hm,
+                "distance_term": (44.9 - 6.55 * math.log10(hb)) * math.log10(d),
+                "metro_correction": C_m,
+            },
+        )