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_api.py

@@ -1,3 +1,4 @@
+from unittest.mock import patch
 from fastapi.testclient import TestClient
 import tempfile
 import os
@@ -14,6 +15,33 @@ from api.main import app  # noqa: E402
 client = TestClient(app)
 
 
+def test_is_ringing_returns_false_when_not_running():
+    """Test that isRinging returns False when no wecker process is running."""
+    headers = {"X-API-Key": "test-secret"}
+    query = """
+    query {
+      isRinging
+    }
+    """
+    res = client.post("/graphql", json={"query": query}, headers=headers)
+    assert res.status_code == 200
+    assert res.json()["data"]["isRinging"] is False
+
+
+@patch("api.schema.is_wecker_ringing", return_value=True)
+def test_is_ringing_returns_true_when_running(mock_is_ringing):
+    """Test that isRinging returns True when a wecker process is running."""
+    headers = {"X-API-Key": "test-secret"}
+    query = """
+    query {
+      isRinging
+    }
+    """
+    res = client.post("/graphql", json={"query": query}, headers=headers)
+    assert res.status_code == 200
+    assert res.json()["data"]["isRinging"] is True
+
+
 def test_auth_missing():
     query = """
     query {
@@ -173,3 +201,13 @@ def test_graphql_workflow():
     # 7. List again is empty
     res = client.post("/graphql", json={"query": query_get}, headers=headers)
     assert res.json()["data"]["getAlarms"] == []
+
+
+def test_pid_file_same_shared_constant_in_api():
+    """Ensure api.schema uses the same PID_FILE from common, not a redefinition."""
+    from api import schema as api_schema
+    import common
+
+    assert api_schema.PID_FILE is common.PID_FILE, (
+        "api.schema.PID_FILE must reference common.PID_FILE, not redefine it"
+    )