fix(salary): skip non-numeric and identifier columns in Excel conversion (#152)

parse_sheet treated every column that was not company/city as a salary category, with no check that the column actually held numeric salary data. This turned free-text columns (e.g. Notes) into bogus string categories and numeric identifier columns (e.g. Id) into mistaken salary indexes.

- Drop identifier headers (ID_PATTERNS = {id, personnummer}) at classification time.

- Skip non-numeric standalone values and fully-null count/index pairs at row-processing time.

- Adds regression tests (skips_free_text_column, skips_numeric_identifier_column, keeps_numeric_salary_column) that fail on master and pass after the fix.

tests/test_convert_salary_excel.py

@@ -104,6 +104,42 @@ class DetectColumnTypeTests(unittest.TestCase):
                     companies[0]["categories"]["salary"], {"index": 105.5}
                 )
 
+    def test_skips_free_text_column(self):
+        # A free-text "Notes" column must not become a bogus salary category.
+        ws = FakeWorksheet([
+            ("Company", "Salary Index", "Notes"),
+            ("Example Corp", 105.5, "good"),
+        ])
+
+        companies = parse_sheet(ws)
+
+        self.assertIn("salary_index", companies[0]["categories"])
+        self.assertNotIn("notes", companies[0]["categories"])
+
+    def test_skips_numeric_identifier_column(self):
+        # A numeric "Id" column (employee id) must not be treated as a salary index.
+        ws = FakeWorksheet([
+            ("Company", "Salary Index", "Id"),
+            ("Example Corp", 105.5, 7),
+        ])
+
+        companies = parse_sheet(ws)
+
+        self.assertIn("salary_index", companies[0]["categories"])
+        self.assertNotIn("id", companies[0]["categories"])
+
+    def test_keeps_numeric_salary_column(self):
+        # A genuine numeric salary column still produces a salary category.
+        ws = FakeWorksheet([
+            ("Company", "Salary Index"),
+            ("Example Corp", 105.5),
+        ])
+
+        companies = parse_sheet(ws)
+
+        self.assertIn("salary_index", companies[0]["categories"])
+        self.assertEqual(companies[0]["categories"]["salary_index"], {"index": 105.5})
+
 
 if __name__ == "__main__":
     unittest.main()