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

README.md

@@ -57,6 +57,14 @@ Instead of manually editing your crontab, this project provides a simple GraphQL
 
 ### Example API Usage
 
+**Check if alarm is currently ringing:**
+```graphql
+query {
+  isRinging
+}
+```
+Returns `true` if the wecker alarm clock is currently active (playing music), `false` otherwise.
+
 **Get all alarms:**
 ```graphql
 query {
@@ -92,6 +100,15 @@ mutation {
 ```
 
 
+### Example response for `isRinging`:
+```json
+{
+  "data": {
+    "isRinging": true
+  }
+}
+```
+
 ## How the Puzzle Works
 1. **Ringing:** The music plays in an endless loop.
 2. **Start:** Press the arcade button once to start the puzzle. Wait 3 seconds.