Add assert-based pytest coverage for recent fixes

- tests/conftest.py: put repo root on sys.path so `import src...` resolves
  under pytest regardless of invocation directory.
- test_matrix_formatting.py: lock in HTML formatted_body + plain-text fallback,
  URL linkification, HTML escaping, and send_alert field rendering.
- test_station_persistence.py: cover default-load, save/reload round-trip
  (incl. Thai text), runtime-file precedence, and atomic-write cleanup.

These are real assert-based tests (unlike the existing print-style scripts) so
CI can gate on them. 13 tests, all passing.

tests/conftest.py

@@ -0,0 +1,12 @@
+"""Shared pytest configuration.
+
+Ensures the repository root is on sys.path so tests can import the ``src``
+package regardless of the working directory pytest is invoked from.
+"""
+
+import os
+import sys
+
+REPO_ROOT = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+if REPO_ROOT not in sys.path:
+    sys.path.insert(0, REPO_ROOT)