feat: add FIT CRC calculation

tests/fit/test_crc.py

@@ -0,0 +1,11 @@
+from app.fit.crc import fit_crc
+
+
+def test_empty_crc_is_zero() -> None:
+    assert fit_crc(b"") == 0
+
+
+def test_crc_is_incremental_equivalent() -> None:
+    payload = b".FIT-device-metadata"
+    assert fit_crc(payload) == fit_crc(memoryview(payload))
+    assert 0 <= fit_crc(payload) <= 0xFFFF