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/schemas.py

@@ -219,9 +219,13 @@ class ProcessingLogResponse(BaseModel):
 
 # Notification Config Schemas
 class NotificationConfigBase(BaseModel):
+    name: str = Field(
+        ..., max_length=255, description="Friendly name for this notification channel"
+    )
     channel: NotificationChannel
+    apprise_url: Optional[str] = Field(None, description="Apprise notification URL")
     is_enabled: bool = True
-    config: Dict[str, Any]
+    config: Dict[str, Any] = Field(default_factory=dict)
     notify_on_errors: bool = True
     notify_on_success: bool = False
     notify_threshold: int = Field(default=3, gt=0, le=100)
@@ -232,6 +236,9 @@ class NotificationConfigCreate(NotificationConfigBase):
 
 
 class NotificationConfigUpdate(BaseModel):
+    name: Optional[str] = Field(None, max_length=255)
+    channel: Optional[NotificationChannel] = None
+    apprise_url: Optional[str] = None
     is_enabled: Optional[bool] = None
     config: Optional[Dict[str, Any]] = None
     notify_on_errors: Optional[bool] = None
@@ -248,6 +255,46 @@ class NotificationConfigResponse(NotificationConfigBase):
     model_config = ConfigDict(from_attributes=True)
 
 
+class NotificationTestRequest(BaseModel):
+    apprise_url: str = Field(..., description="Apprise URL to test")
+
+
+class NotificationTestResponse(BaseModel):
+    success: bool
+    message: str
+
+
+# Admin Notification Config Schemas
+class AdminNotificationConfigBase(BaseModel):
+    name: str = Field(..., max_length=255)
+    apprise_url: str = Field(..., description="Apprise notification URL")
+    is_enabled: bool = True
+    notify_on_errors: bool = True
+    notify_on_system_events: bool = True
+    description: Optional[str] = None
+
+
+class AdminNotificationConfigCreate(AdminNotificationConfigBase):
+    pass
+
+
+class AdminNotificationConfigUpdate(BaseModel):
+    name: Optional[str] = Field(None, max_length=255)
+    apprise_url: Optional[str] = None
+    is_enabled: Optional[bool] = None
+    notify_on_errors: Optional[bool] = None
+    notify_on_system_events: Optional[bool] = None
+    description: Optional[str] = None
+
+
+class AdminNotificationConfigResponse(AdminNotificationConfigBase):
+    id: int
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = ConfigDict(from_attributes=True)
+
+
 # Subscription Schemas
 class SubscriptionPlanResponse(BaseModel):
     id: int