fix: correct xAI TTS pricing to $15.00/1M characters (#3089)

xAI's Voice pricing table lists Text to Speech at $15.00 / 1M characters,
but we were charging $4.20 / 1M characters (420 microcents/char), undercharging
by ~72%. Update the cost constant, listEngines pricing, and test expectation
to 1500 microcents/char.

src/backend/drivers/ai-tts/providers/xai/XAITTSProvider.test.ts

@@ -113,7 +113,7 @@ describe('XAITTSProvider catalog', () => {
         expect(engines[0]).toMatchObject({
             id: 'xai-tts',
             provider: 'xai',
-            pricing_per_million_chars: 420,
+            pricing_per_million_chars: 1500,
         });
     });
 });

src/backend/drivers/ai-tts/providers/xai/XAITTSProvider.ts

@@ -79,7 +79,7 @@ export class XAITTSProvider extends TTSProvider {
                 id: 'xai-tts',
                 name: 'xAI TTS',
                 provider: 'xai',
-                pricing_per_million_chars: 420,
+                pricing_per_million_chars: 1500,
             },
         ];
     }

src/backend/drivers/ai-tts/providers/xai/costs.ts

@@ -17,18 +17,10 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
-// xAI TTS pricing: $4.20 per 1M characters
-// = 420 cents per 1M characters
-// = 0.00042 cents per character
-// = 0.42 microcents per character
-// We store microcents per character (cents * 1_000_000 / 1_000_000 chars).
-// $4.20 / 1M chars = 420 cents / 1M chars = 0.00042 cents/char = 0.42 ucents/char
-// But metering uses integer microcents, so we round up to 1 microcent per char
-// to avoid undercharging. More precisely: 420 * 1_000_000 / 1_000_000 = 420 ucents per char? No.
-// Let's be precise:
-// $4.20 per 1M chars = 420 cents per 1M chars
-// In microcents: 420 * 1_000_000 = 420_000_000 microcents per 1M chars
-// Per character: 420_000_000 / 1_000_000 = 420 microcents per character
+// xAI TTS pricing (per xAI docs, Voice pricing table): $15.00 per 1M characters
+// $15.00 per 1M chars = 1500 cents per 1M chars
+// In microcents: 1500 * 1_000_000 = 1_500_000_000 microcents per 1M chars
+// Per character: 1_500_000_000 / 1_000_000 = 1500 microcents per character
 export const XAI_TTS_COSTS: Record<string, number> = {
-    'xai-tts': 420,
+    'xai-tts': 1500,
 };