feat: multi-domain failover for provider accounts

Each ProviderAccount can now have multiple base URLs (provider_urls table).
On stream failure, BroadcastGroup cycles to the next domain immediately
with no wait; backs off only after all domains have been tried once.
Background health checker pings every domain every 5 min via player_api.php
and updates status/response_ms. Admin UI shows domain list with color-coded
status badges and a "Verificar todos" button per provider.

backend/main.py

@@ -14,6 +14,7 @@ from .config import settings
 from .database import init_db, AsyncSessionLocal
 from .core.pool import pool
 from .core.epg_manager import epg_background_task
+from .core.health_checker import health_check_loop
 from .database import get_db
 
 logging.basicConfig(
@@ -67,13 +68,15 @@ async def lifespan(app: FastAPI):
     await pool.init()
     epg_task = asyncio.create_task(epg_background_task(get_db))
     auto_sync_task = asyncio.create_task(_auto_sync_loop())
+    health_task = asyncio.create_task(health_check_loop())
     logger.info("KiraStream ready")
     yield
     logger.info("KiraStream shutting down...")
     await pool.close()
     epg_task.cancel()
     auto_sync_task.cancel()
-    for t in (epg_task, auto_sync_task):
+    health_task.cancel()
+    for t in (epg_task, auto_sync_task, health_task):
         try:
             await t
         except asyncio.CancelledError: