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/core/pool.py

@@ -112,9 +112,10 @@ class ProviderPool:
             provider_name = next(
                 (pm.get("provider_name", "") for pm in provider_maps if pm["provider_account_id"] == account_id), ""
             )
+            stream_urls = await self._build_stream_urls(account_id, stream_url)
             group = BroadcastGroup(
                 channel_id=channel_id,
-                stream_url=stream_url,
+                stream_urls=stream_urls,
                 provider_account_id=account_id,
                 channel_name=channel_name,
                 provider_name=provider_name,
@@ -239,6 +240,43 @@ class ProviderPool:
         rest = [pm for pm in provider_maps if pm["provider_account_id"] not in pref_set]
         return preferred + rest
 
+    async def _build_stream_urls(self, account_id: int, primary_stream_url: str) -> list[str]:
+        """Build list of stream URLs for failover: one per active ProviderUrl, substituting base URL."""
+        from urllib.parse import urlparse, urlunparse
+        from ..models.provider import ProviderUrl
+        from ..database import AsyncSessionLocal
+        from sqlalchemy import select
+
+        parsed = urlparse(primary_stream_url)
+
+        async with AsyncSessionLocal() as db:
+            result = await db.execute(
+                select(ProviderUrl)
+                .where(
+                    ProviderUrl.provider_account_id == account_id,
+                    ProviderUrl.is_active == True,  # noqa: E712
+                )
+                .order_by(ProviderUrl.priority)
+            )
+            provider_urls = result.scalars().all()
+
+        if not provider_urls:
+            return [primary_stream_url]
+
+        urls = []
+        for pu in provider_urls:
+            alt = urlparse(pu.url.rstrip('/'))
+            reconstructed = urlunparse((
+                alt.scheme or parsed.scheme,
+                alt.netloc,
+                parsed.path,
+                parsed.params,
+                parsed.query,
+                parsed.fragment,
+            ))
+            urls.append(reconstructed)
+        return urls
+
     async def _find_free_slot(self, provider_maps: list[dict]) -> tuple[int | None, str | None]:
         for pm in provider_maps:
             account_id = pm["provider_account_id"]