Replace pandas with a small openpyxl/csv reader for imports.

pandas + numpy were ~60MB of the shipped venv for three call sites (alarm
Excel import, managed-NE import, import template). netx_api/tabular.py
reads/writes xlsx and csv and parses spreadsheet timestamps.

Checked against the pandas version on the same inputs: timestamp parsing
identical for 24 formats, managed-NE csv/xlsx import, stored rows and both
templates identical. Differences, all fixes:
- alarm codes in mixed text/number columns are "1001", not "1001.0"
- error row numbers are the real sheet rows (blank rows no longer shift them)
- GBK-encoded CSVs (Chinese Excel "Save as CSV") import instead of failing
- "2024年1月2日 03:04:05" style timestamps parse

Also: conftest compares parsed DB URLs (newer SQLAlchemy renders "C:" as
"C%3A").

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/conftest.py

@@ -15,9 +15,12 @@ os.environ["NETX_CREDENTIAL_SECRET_FILE"] = str(Path(_TEST_STATE.name) / "creden
 
 
 def pytest_sessionstart(session):
+    from sqlalchemy.engine import make_url
+
     from netx_api.db import engine
 
-    if str(engine.url) != _TEST_DATABASE_URL:
+    # Compare parsed URLs: newer SQLAlchemy renders "C:" as "C%3A" in str(url).
+    if engine.url != make_url(_TEST_DATABASE_URL):
         raise pytest.UsageError("NetX tests require the isolated temporary database")