Add initial backend structure with models, utilities, and settings management

- Introduced .gitignore to exclude common Python artifacts and IDE files.
- Added contour models for contour generation.
- Implemented cryptographic utilities for data encryption and decryption.
- Created CSV metadata model for handling wafer data.
- Developed time-indexed data segment model.
- Established local settings model for application configuration.
- Added Z-wafer data models and parser for CSV file handling.
- Implemented Marching Squares algorithm for contour generation.

backend/contour_models.py

@@ -0,0 +1,26 @@
+from dataclasses import dataclass, field
+from typing import List
+
+
+# ===== Single Contour Segment =====
+@dataclass
+class ContourSegment:
+    start_x: float
+    start_y: float
+    end_x: float
+    end_y: float
+
+    @property
+    def start(self):
+        return (self.start_x, self.start_y)
+
+    @property
+    def end(self):
+        return (self.end_x, self.end_y)
+
+
+# ===== Contour Line =====
+@dataclass
+class ContourLine:
+    level: float
+    segments: List[ContourSegment] = field(default_factory=list)