Cost dashboard: TTS providers endpoint, header link, polish

Adds /api/costs/tts endpoint backed by get_tts_providers() in cost_db,
surfaces a Costs link in the control panel header, and small polish
to the costs page HTML/JS.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/main.py

@@ -10354,6 +10354,11 @@ async def get_cost_expensive_calls(period: str = "all", limit: int = 10):
     return cost_db.get_expensive_calls(period, limit)
 
 
+@app.get("/api/costs/tts")
+async def get_cost_tts_providers(period: str = "all"):
+    return cost_db.get_tts_providers(period)
+
+
 # --- Caller Screening ---
 
 SCREENING_PROMPT = """You are a friendly, brief phone screener for "Luke at the Roost" radio show.