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/test_matrix_formatting.py

@@ -0,0 +1,98 @@
+"""Assert-based tests for Matrix message formatting.
+
+Matrix clients only render formatting from an HTML ``formatted_body``; Markdown
+in the plain ``body`` shows as literal characters. These tests lock in that the
+notifier emits real HTML plus a clean plain-text fallback, and that untrusted
+station data is HTML-escaped.
+"""
+
+import datetime
+
+from src.alerting import AlertLevel, MatrixNotifier, WaterAlert, markdown_to_matrix_html, strip_markdown
+
+
+class _FakeResponse:
+    def raise_for_status(self):
+        pass
+
+    def json(self):
+        return {"event_id": "$test"}
+
+
+def _notifier_capturing(captured):
+    """A MatrixNotifier whose HTTP PUT records the JSON payload into ``captured``."""
+    notifier = MatrixNotifier("https://hs.example", "token", "!room:hs.example")
+
+    def fake_put(url, headers=None, json=None, timeout=None):
+        captured.update(json)
+        return _FakeResponse()
+
+    notifier.session.put = fake_put
+    return notifier
+
+
+def test_bold_becomes_strong():
+    assert markdown_to_matrix_html("**hi**") == "<strong>hi</strong>"
+
+
+def test_url_is_linkified():
+    out = markdown_to_matrix_html("see https://x.example/z")
+    assert '<a href="https://x.example/z">https://x.example/z</a>' in out
+
+
+def test_newlines_become_br():
+    assert markdown_to_matrix_html("a\nb") == "a<br/>b"
+
+
+def test_html_is_escaped():
+    out = markdown_to_matrix_html("<script> & 'stuff'")
+    assert "&lt;script&gt;" in out
+    assert "&amp;" in out
+    assert "<script>" not in out
+
+
+def test_strip_markdown_removes_bold_markers():
+    assert strip_markdown("**WATER LEVEL ALERT**") == "WATER LEVEL ALERT"
+
+
+def test_send_message_sends_html_and_plain_fallback():
+    captured = {}
+    notifier = _notifier_capturing(captured)
+
+    assert notifier.send_message("**hi** http://x.example") is True
+    assert captured["format"] == "org.matrix.custom.html"
+    assert "<strong>hi</strong>" in captured["formatted_body"]
+    # Plain body has the markdown markers stripped.
+    assert captured["body"] == "hi http://x.example"
+
+
+def test_send_message_plain_when_markdown_disabled():
+    captured = {}
+    notifier = _notifier_capturing(captured)
+
+    assert notifier.send_message("**raw**", markdown=False) is True
+    assert "formatted_body" not in captured
+    assert captured["body"] == "**raw**"
+
+
+def test_send_alert_renders_alert_fields():
+    captured = {}
+    notifier = _notifier_capturing(captured)
+    alert = WaterAlert(
+        station_code="P.1",
+        station_name="สะพานนวรัฐ",
+        alert_type="Zone 7 - Critical",
+        level=AlertLevel.CRITICAL,
+        water_level=4.62,
+        threshold=4.60,
+        discharge=612.0,
+        timestamp=datetime.datetime(2026, 7, 22, 14, 30, 0),
+    )
+
+    assert notifier.send_alert(alert) is True
+    html = captured["formatted_body"]
+    assert "<strong>WATER LEVEL ALERT</strong>" in html
+    assert "สะพานนวรัฐ" in html  # Thai station name preserved
+    assert "<strong>Current Level:</strong>" in html
+    # Plain fallback carries no leftover markdown markers.
+    assert "**" not in captured["body"]