Add isRinging query to check alarm state via API

Features:
- New GraphQL query 'isRinging' returns true/false if wecker is active
- Checks the wecker process via PID file (PID_FILE in common.py)

DRY refactoring:
- Extract PID_FILE into shared common.py module
- Both wecker.py and api/schema.py import from common
- DRY enforcement tests verify identity (is) not just equality

Tests:
- test_is_ringing_returns_false_when_not_running
- test_is_ringing_returns_true_when_running
- test_pid_file_defined_once_across_modules (DRY enforcement)
- test_pid_file_same_shared_constant_in_api (DRY enforcement)
- Cleaned up unused imports in test_single_instance.py

Docs:
- Updated README.md with isRinging query documentation

tests/test_single_instance.py

@@ -1,7 +1,7 @@
-import os
-import sys
 import pytest
-from unittest.mock import MagicMock, patch
+from unittest.mock import patch
+
+import common
 import wecker
 
 # We need to mock the PID file existence and os.kill to test the logic
@@ -80,3 +80,17 @@ def test_remove_pid_file_wrong_pid(mock_pid_file):
         
         wecker.remove_pid_file()
         assert mock_pid_file.exists()
+
+
+def test_pid_file_defined_once_across_modules():
+    """
+    DRY principle: PID_FILE must be defined in common.py and imported by
+    both wecker.py and api/schema.py — not redefined in each.
+    """
+    # wecker.py imports PID_FILE from common — verify it's the same object
+    assert wecker.PID_FILE is common.PID_FILE, (
+        "wecker.PID_FILE must reference common.PID_FILE, not redefine it"
+    )
+
+    # Verify the path ends with "wecker.pid"
+    assert common.PID_FILE.endswith("wecker.pid")