fixed error in get_thunderstorms: gemeinde -> gemeindenr

[a7rk]

API returns gemeindenr not gemeinde

[tklecka]

Fix is right — the live endpoint returns gemeindenr, so get_thunderstorms() was failing on every call.

Two things before merge:

1. tests/test_client.py still uses "gemeinde" in its fixtures, so test_get_thunderstorms fails with this patch (Could not parse thunderstorm response: 'gemeindenr'). CI hasn't reported on this PR, so it went unnoticed.

2. test_get_thunderstorms_malformed is meant to cover a missing intensitaet, but now raises on the missing gemeindenr instead. Needs {"gemeindenr": 1} to keep testing what it intends.

Minor: .gitignore lost its trailing newline.

[tklecka]

Tests updated, suite is green (20 passed), ruff and mypy clean. Thanks!

Only leftover is the missing trailing newline in .gitignore — not worth another round-trip, I'll fix it after merge.