Add Apprise alerting capabilities (backend)

- Add name + apprise_url columns to NotificationConfig model
- Add AdminNotificationConfig model for system-wide alert channels
- Create NotificationService with send_user_notification,
  send_admin_notification, and test_notification helpers
- Replace stub notifications.py with full CRUD + /test endpoint
- Add admin notification config CRUD + /test to admin.py
- Update NotificationConfig schemas: name required, apprise_url
  optional, config optional (default {})
- Add AdminNotificationConfig* schemas
- Integrate send_user_notification into process_mail_account task
  for Gmail revocation, forwarding failures, and exceptions
- Update CHANGELOG.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

backend/app/models/database_models.py

@@ -289,6 +289,11 @@ class NotificationConfig(Base):
         Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False
     )
 
+    name = Column(String(255), nullable=False, default="My Notification")
+    apprise_url = Column(
+        Text, nullable=True
+    )  # The Apprise URL e.g. tgram://token/chatid
+
     # Channel details
     channel: Column[str] = Column(SQLEnum(NotificationChannel), nullable=False)
     is_enabled = Column(Boolean, default=True)
@@ -590,3 +595,29 @@ class AppSetting(Base):
         onupdate=lambda: datetime.now(timezone.utc),
         nullable=False,
     )
+
+
+class AdminNotificationConfig(Base):
+    """System-wide admin notification channels"""
+
+    __tablename__ = "admin_notification_configs"
+
+    id = Column(Integer, primary_key=True, index=True)
+    name = Column(String(255), nullable=False)
+    apprise_url = Column(Text, nullable=False)
+    is_enabled = Column(Boolean, default=True)
+    notify_on_errors = Column(Boolean, default=True)
+    notify_on_system_events = Column(Boolean, default=True)
+    description = Column(Text, nullable=True)
+
+    created_at = Column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc),
+        nullable=False,
+    )
+    updated_at = Column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc),
+        onupdate=lambda: datetime.now(timezone.utc),
+        nullable=False,
+    )