Add email notification settings management

Implemented backend and frontend support for managing SMTP settings for email notifications. Includes API endpoints, database migration, and UI integration for configuring and testing email alerts.

backend/app/schemas/admin_settings.py

@@ -0,0 +1,48 @@
+from datetime import datetime
+
+from pydantic import BaseModel, EmailStr, field_validator, model_validator
+
+
+class EmailSettingsOut(BaseModel):
+    enabled: bool
+    smtp_host: str | None
+    smtp_port: int
+    smtp_username: str | None
+    from_email: EmailStr | None
+    use_starttls: bool
+    use_ssl: bool
+    alert_recipients: list[EmailStr]
+    has_password: bool
+    updated_at: datetime | None
+
+
+class EmailSettingsUpdate(BaseModel):
+    enabled: bool = False
+    smtp_host: str | None = None
+    smtp_port: int = 587
+    smtp_username: str | None = None
+    smtp_password: str | None = None
+    clear_smtp_password: bool = False
+    from_email: EmailStr | None = None
+    use_starttls: bool = True
+    use_ssl: bool = False
+    alert_recipients: list[EmailStr] = []
+
+    @field_validator("smtp_port")
+    @classmethod
+    def validate_port(cls, value: int) -> int:
+        if value < 1 or value > 65535:
+            raise ValueError("smtp_port must be between 1 and 65535")
+        return value
+
+    @model_validator(mode="after")
+    def validate_tls_combo(self):
+        if self.use_starttls and self.use_ssl:
+            raise ValueError("use_starttls and use_ssl cannot both be true")
+        return self
+
+
+class EmailSettingsTestRequest(BaseModel):
+    recipient: EmailStr
+    subject: str = "NexaPG test notification"
+    message: str = "This is a test alert notification from NexaPG."