fix: Pin NumPy < 2.0 to fix macOS ARM backend crash

- Pin numpy<2.0 in requirements.txt to avoid NumPy 2.x breaking changes
- Add NumPy hidden imports to backend_server.spec for PyInstaller compatibility
- Fix requirements.txt encoding (UTF-16 → UTF-8)

Fixes: Backend crashes with 'ModuleNotFoundError: No module named numpy._core._exceptions'
on macOS Apple Silicon (M1/M2/M3/M4) due to NumPy 2.x internal module restructuring
that PyInstaller doesn't properly bundle.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Mayank Pandey

.gitignore

@@ -226,4 +226,14 @@ backend_server-x86_64-unknown-linux-gnu
 backend_server-aarch64-unknown-linux-gnu
 backend_server-aarch64-apple-darwin
 backend_server-x86_64-apple-darwin
-backend_server-i686-unknown-linux-gnu
+backend_server-i686-unknown-linux-gnu
+
+# Keep only the placeholder in the source-controlled backend bundle directory.
+/frontend/src-tauri/backend_server_bundle/*
+!/frontend/src-tauri/backend_server_bundle/.gitkeep
+
+# Ignore generated package metadata/data files from bundled Python environments.
+**/*.dist-info/
+**/RECORD
+**/METADATA
+**/rg_cities1000.csv

backend/backend_server.spec

@@ -33,6 +33,12 @@ a = Analysis(
         'multiprocessing.synchronize',
         'multiprocessing.resource_tracker',
         '_multiprocessing',
+        # NumPy hidden imports for PyInstaller compatibility
+        'numpy._core._exceptions',
+        'numpy._core._methods',
+        'numpy._core._dtype_ctypes',
+        'numpy._core._internal',
+        'numpy._core.multiarray',
     ],
     hookspath=[],
     hooksconfig={},