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/models/provider.py

@@ -1,4 +1,4 @@
-from sqlalchemy import Column, Integer, String, Boolean, DateTime, Text, func
+from sqlalchemy import Column, Integer, String, Boolean, DateTime, Text, ForeignKey, func
 from sqlalchemy.orm import relationship
 from ..database import Base
 
@@ -22,3 +22,20 @@ class ProviderAccount(Base):
     created_at = Column(DateTime(timezone=True), server_default=func.now())
 
     channel_maps = relationship("ChannelProviderMap", back_populates="provider_account", cascade="all, delete-orphan")
+    extra_urls = relationship("ProviderUrl", back_populates="provider_account", cascade="all, delete-orphan", order_by="ProviderUrl.priority")
+
+
+class ProviderUrl(Base):
+    __tablename__ = "provider_urls"
+
+    id = Column(Integer, primary_key=True, index=True)
+    provider_account_id = Column(Integer, ForeignKey("provider_accounts.id", ondelete="CASCADE"), nullable=False, index=True)
+    url = Column(String(500), nullable=False)
+    priority = Column(Integer, default=0, nullable=False)  # 0=primary, higher=lower priority
+    is_active = Column(Boolean, default=True, nullable=False)
+    status = Column(String(20), default="unknown", nullable=False)  # unknown|ok|error|timeout
+    response_ms = Column(Integer, nullable=True)
+    last_checked_at = Column(DateTime(timezone=True), nullable=True)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+
+    provider_account = relationship("ProviderAccount", back_populates="extra_urls")