Landing page: testimonials slider, how-it-works page, 25 TTS voices

- Add testimonial slider with 8 fake caller reviews
- Add how-it-works page with visual architecture diagram
- Expand voice pools: Inworld 25 voices (14M/11F), ElevenLabs 22 (14M/8F)
- Voice pools auto-switch when TTS provider changes
- Add cover art locally, update cache-busted image refs
- Add "More from Luke" footer links (MMG, prints, YouTube)
- Ad channel configurable in settings UI

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/services/audio.py

@@ -135,6 +135,7 @@ class AudioService:
         live_caller_channel: Optional[int] = None,
         music_channel: Optional[int] = None,
         sfx_channel: Optional[int] = None,
+        ad_channel: Optional[int] = None,
         phone_filter: Optional[bool] = None
     ):
         """Configure audio devices and channels"""
@@ -152,6 +153,8 @@ class AudioService:
             self.music_channel = music_channel
         if sfx_channel is not None:
             self.sfx_channel = sfx_channel
+        if ad_channel is not None:
+            self.ad_channel = ad_channel
         if phone_filter is not None:
             self.phone_filter = phone_filter
 
@@ -168,6 +171,7 @@ class AudioService:
             "live_caller_channel": self.live_caller_channel,
             "music_channel": self.music_channel,
             "sfx_channel": self.sfx_channel,
+            "ad_channel": self.ad_channel,
             "phone_filter": self.phone_filter,
         }