Change default sample rate to 48kHz and add validation

Changes:
- Update default sample rate from 44.1kHz to 48kHz (more professional standard)
- Add sample rate validation in play_sound_file()
- Display clear warning when uploaded file doesn't match configured rate
- Update default sound generation (dial tone & beep) to use configured RATE
- Prevent playback of incompatible sample rates (avoids ALSA errors)

Error Handling:
- Check file sample rate before attempting playback
- Print warning with actual vs expected sample rate
- Return False and skip playback if mismatch detected
- User-friendly error message with resampling instructions

This fixes the "Invalid sample rate" error when playing files with
non-matching sample rates. Users must now resample their audio files
to match the configured rate (default 48kHz).

To resample audio files:
ffmpeg -i input.wav -ar 48000 output.wav

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

config.example.json

@@ -11,7 +11,7 @@
     "chunk_size": 1024,
     "format": "paInt16",
     "channels": 1,
-    "sample_rate": 44100,
+    "sample_rate": 48000,
     "max_record_seconds": 300
   },
   "paths": {

rotary_phone_web.py

@@ -354,35 +354,33 @@ class RotaryPhone:
     
     def generate_default_dialtone(self):
         """Generate a classic dial tone (350Hz + 440Hz) and save as default"""
-        print("Generating default dial tone...")
+        print(f"Generating default dial tone at {RATE}Hz...")
         duration = 3
-        sample_rate = 44100
-        t = np.linspace(0, duration, int(sample_rate * duration), False)
-        
+        t = np.linspace(0, duration, int(RATE * duration), False)
+
         # Generate two frequencies and combine them
         tone1 = np.sin(2 * np.pi * 350 * t)
         tone2 = np.sin(2 * np.pi * 440 * t)
         tone = (tone1 + tone2) / 2
-        
+
         # Convert to 16-bit PCM
         tone = (tone * 32767).astype(np.int16)
-        
+
         # Save as WAV file
         wf = wave.open(DIALTONE_FILE, 'wb')
         wf.setnchannels(1)
         wf.setsampwidth(2)  # 16-bit
-        wf.setframerate(sample_rate)
+        wf.setframerate(RATE)
         wf.writeframes(tone.tobytes())
         wf.close()
-        print(f"Default dial tone saved to {DIALTONE_FILE}")
+        print(f"Default dial tone saved to {DIALTONE_FILE} ({RATE}Hz, 16-bit)")
 
     def generate_default_beep(self):
         """Generate a simple beep tone (1000Hz) to indicate recording start"""
-        print("Generating default beep sound...")
+        print(f"Generating default beep sound at {RATE}Hz...")
         beep_file = os.path.join(SOUNDS_DIR, "beep.wav")
         duration = 0.5  # Half second beep
-        sample_rate = 44100
-        t = np.linspace(0, duration, int(sample_rate * duration), False)
+        t = np.linspace(0, duration, int(RATE * duration), False)
 
         # Generate 1000Hz tone
         frequency = 1000
@@ -395,10 +393,10 @@ class RotaryPhone:
         wf = wave.open(beep_file, 'wb')
         wf.setnchannels(1)
         wf.setsampwidth(2)  # 16-bit
-        wf.setframerate(sample_rate)
+        wf.setframerate(RATE)
         wf.writeframes(tone.tobytes())
         wf.close()
-        print(f"Default beep sound saved to {beep_file}")
+        print(f"Default beep sound saved to {beep_file} ({RATE}Hz, 16-bit)")
 
     def play_sound_file(self, filepath, check_hook_status=True):
         """Play a WAV file with volume control
@@ -417,11 +415,19 @@ class RotaryPhone:
         try:
             wf = wave.open(filepath, 'rb')
 
+            # Check sample rate compatibility
+            file_rate = wf.getframerate()
+            if file_rate != RATE:
+                print(f"WARNING: File sample rate ({file_rate}Hz) doesn't match configured rate ({RATE}Hz)")
+                print(f"Audio may not play correctly. Please resample the file to {RATE}Hz")
+                wf.close()
+                return False
+
             # Use configured audio device
             stream = self.audio.open(
                 format=self.audio.get_format_from_width(wf.getsampwidth()),
                 channels=wf.getnchannels(),
-                rate=wf.getframerate(),
+                rate=file_rate,
                 output=True,
                 output_device_index=AUDIO_DEVICE_INDEX,
                 frames_per_buffer=CHUNK