Separate events and notifications into focused modules

The events.rs file mixed three domains: device-event recording, change
detection, and the entire notification pipeline (rendering + delivery +
sending). This made it long, gave functions side effects beyond their
stated goal (classify_* silently recorded events), and intertwined the
events and notifications logic.

Split along domain boundaries:
- model::device_events now owns DeviceChange, the shared contract.
- events records device events only (record_new_device/record_known_device);
  events/detection.rs holds pure change detection.
- new notifications module owns rendering, delivery, and sending
  (notifications.rs + delivery.rs + render.rs); pushover/error moved here.

Data now flows one way: events produces DeviceChange, notifications
consumes it, both depend only on model. Scanners/pipeline/main orchestrate.
classify_* renamed to record_* so the write is the stated goal; send and
send_notification collapsed into persist_and_deliver.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/notifications/error.rs

@@ -0,0 +1,30 @@
+use std::{error, fmt};
+
+#[derive(Debug)]
+pub enum DeliveryError {
+    ParsePushover(pushover::Error),
+}
+
+impl fmt::Display for DeliveryError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            DeliveryError::ParsePushover(..) => {
+                write!(f, "Error delivering notification via Pushover")
+            }
+        }
+    }
+}
+
+impl error::Error for DeliveryError {
+    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
+        match *self {
+            DeliveryError::ParsePushover(ref e) => Some(e),
+        }
+    }
+}
+
+impl From<pushover::Error> for DeliveryError {
+    fn from(err: pushover::Error) -> DeliveryError {
+        DeliveryError::ParsePushover(err)
+    }
+}