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

api/schema.py

@@ -4,6 +4,7 @@ import os
 import sys
 from pathlib import Path
 from api.crontab_manager import CrontabManager
+from common import PID_FILE
 
 
 def get_manager():
@@ -11,6 +12,20 @@ def get_manager():
     return CrontabManager(tabfile=tabfile)
 
 
+def is_wecker_ringing() -> bool:
+    """Check if the wecker process is currently running via the PID file."""
+    if not os.path.exists(PID_FILE):
+        return False
+    try:
+        pid: int
+        with open(PID_FILE) as f:
+            pid = int(f.read().strip())
+        os.kill(pid, 0)
+        return True
+    except (ValueError, ProcessLookupError, PermissionError, OSError):
+        return False
+
+
 @strawberry.type
 class Alarm:
     id: str
@@ -21,6 +36,10 @@ class Alarm:
 
 @strawberry.type
 class Query:
+    @strawberry.field
+    def is_ringing(self) -> bool:
+        return is_wecker_ringing()
+
     @strawberry.field
     def get_alarms(self) -> List[Alarm]:
         manager = get_manager()