Add customizable email templates and remove alert recipients

Replaced the fixed `alert_recipients` list with customizable subject and body templates for alerts and warnings. This allows for more flexible and dynamic email notifications using placeholder variables. Updated relevant backend and frontend components to support this feature.

backend/app/schemas/admin_settings.py

@@ -12,7 +12,10 @@ class EmailSettingsOut(BaseModel):
     from_email: EmailStr | None
     use_starttls: bool
     use_ssl: bool
-    alert_recipients: list[EmailStr]
+    warning_subject_template: str | None
+    alert_subject_template: str | None
+    warning_body_template: str | None
+    alert_body_template: str | None
     has_password: bool
     updated_at: datetime | None
 
@@ -28,7 +31,10 @@ class EmailSettingsUpdate(BaseModel):
     from_email: EmailStr | None = None
     use_starttls: bool = True
     use_ssl: bool = False
-    alert_recipients: list[EmailStr] = []
+    warning_subject_template: str | None = None
+    alert_subject_template: str | None = None
+    warning_body_template: str | None = None
+    alert_body_template: str | None = None
 
     @field_validator("smtp_port")
     @classmethod