@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/itu_r_p1546.py

@@ -0,0 +1,74 @@
+"""
+ITU-R P.1546 model for point-to-area predictions.
+
+Valid for:
+- Frequency: 30-3000 MHz
+- Distance: 1-1000 km
+- Time percentages: 1%, 10%, 50%
+
+Best for: VHF/UHF broadcasting and land mobile services.
+Reference: ITU-R P.1546-6 (2019)
+"""
+
+import math
+from app.propagation.base import PropagationModel, PropagationInput, PropagationOutput
+
+
+class ITUR_P1546Model(PropagationModel):
+    """
+    Simplified P.1546 implementation.
+
+    Full implementation would include terrain clearance angle,
+    mixed path (land/sea), and time variability.
+    """
+
+    @property
+    def name(self) -> str:
+        return "ITU-R-P.1546"
+
+    @property
+    def frequency_range(self) -> tuple:
+        return (30, 3000)
+
+    @property
+    def distance_range(self) -> tuple:
+        return (1000, 1000000)  # 1-1000 km
+
+    def calculate(self, input: PropagationInput) -> PropagationOutput:
+        f = input.frequency_mhz
+        d = max(input.distance_m / 1000, 1.0)  # km
+        h1 = max(input.tx_height_m, 1.0)
+
+        # Nominal frequency bands
+        if f < 100:
+            f_nom = 100
+        elif f < 600:
+            f_nom = 600
+        else:
+            f_nom = 2000
+
+        # Basic field strength at 1 kW ERP (from curves, simplified regression)
+        E_ref = 106.9 - 20 * math.log10(d)  # dBuV/m at 1kW
+
+        # Height gain for transmitter
+        delta_h1 = 20 * math.log10(h1 / 10) if h1 > 10 else 0
+
+        # Frequency correction
+        delta_f = 20 * math.log10(f / f_nom)
+
+        # Convert field strength to path loss
+        # L = 139.3 - E + 20*log10(f) (for 50 Ohm)
+        E = E_ref + delta_h1 - delta_f
+        L = 139.3 - E + 20 * math.log10(f)
+
+        return PropagationOutput(
+            path_loss_db=L,
+            model_name=self.name,
+            is_los=d < 5,
+            breakdown={
+                "reference_field": E_ref,
+                "height_gain": delta_h1,
+                "frequency_correction": delta_f,
+                "path_loss": L,
+            },
+        )