fix(notifications): lower precipitation intensity threshold for wet detection (#567)

[Orinks]

Summary

• Adds a precipitation_sensitivity setting (light / moderate / heavy) that controls the minimum intensity (mm/h) required to count a minutely data point as wet
• Previously is_wet() used > 0, which triggers on any non-zero value including sensor noise and sub-drizzle traces — now defaults to light (≥ 0.01 mm/h) which still catches light rain while filtering noise
• Thresholds: light ≥ 0.01 mm/h, moderate ≥ 0.1 mm/h, heavy ≥ 1.0 mm/h
• Setting exposed in Settings → Notifications as a dropdown after the start/stop checkboxes
• All existing behaviour preserved (threshold defaults to 0.0 when not set, and light is the default sensitivity)

Changes

• notifications/minutely_precipitation.py: add INTENSITY_THRESHOLD_* constants and SENSITIVITY_THRESHOLDS map; update is_wet(), detect_minutely_precipitation_transition(), and build_minutely_transition_signature() to accept a threshold= kwarg
• models/config.py: add precipitation_sensitivity: str = "light" field with round-trip serialisation and validate_on_access() guard
• notifications/notification_event_manager.py: thread the setting through _check_minutely_precipitation_transition()
• ui/dialogs/settings_tabs/notifications.py: add sensitivity dropdown with three labelled choices
• Tests for is_wet() with each threshold, transition detection with threshold, manager honouring sensitivity, and round-trip/default/validation for the new setting

Test plan

• pytest tests/test_notification_event_manager.py — all 39 tests pass
• pytest tests/test_models.py — all model tests pass including new sensitivity tests
• Full suite: pytest -n auto -m "not integration" — 3253 passed, 4 skipped
• ruff check --fix . && ruff format . — clean

Closes #567