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/services/notification_service.py

@@ -0,0 +1,143 @@
+"""
+Notification service using Apprise for multi-channel alerting.
+
+Supports:
+- User-specific notifications (per-user Apprise URLs)
+- Admin-wide system notifications (system-level alerts)
+- Test notifications to verify configuration
+"""
+
+import logging
+
+import apprise
+
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy import select
+
+from app.models.database_models import NotificationConfig, AdminNotificationConfig
+
+logger = logging.getLogger(__name__)
+
+
+async def send_user_notification(
+    db: AsyncSession,
+    user_id: int,
+    title: str,
+    body: str,
+    notify_on_error: bool = True,
+) -> int:
+    """
+    Send a notification to all enabled notification channels for a given user.
+
+    Args:
+        db: Database session
+        user_id: The user to notify
+        title: Notification title/subject
+        body: Notification body text
+        notify_on_error: If True, only sends to channels with notify_on_errors=True
+                         If False, only sends to channels with notify_on_success=True
+
+    Returns:
+        Number of channels notified successfully
+    """
+    result = await db.execute(
+        select(NotificationConfig).where(
+            NotificationConfig.user_id == user_id,
+            NotificationConfig.is_enabled == True,  # noqa: E712
+            NotificationConfig.apprise_url.isnot(None),
+        )
+    )
+    configs = result.scalars().all()
+
+    if not configs:
+        return 0
+
+    sent = 0
+    for config in configs:
+        if notify_on_error and not config.notify_on_errors:
+            continue
+        if not notify_on_error and not config.notify_on_success:
+            continue
+
+        try:
+            success = await _send_apprise(str(config.apprise_url), title, body)
+            if success:
+                sent += 1
+        except Exception as exc:
+            logger.warning(
+                "Failed to send notification via channel %s (user %s): %s",
+                config.id,
+                user_id,
+                exc,
+            )
+
+    return sent
+
+
+async def send_admin_notification(
+    db: AsyncSession,
+    title: str,
+    body: str,
+) -> int:
+    """
+    Send a notification to all enabled admin notification channels.
+
+    Returns:
+        Number of channels notified successfully
+    """
+    result = await db.execute(
+        select(AdminNotificationConfig).where(
+            AdminNotificationConfig.is_enabled == True,  # noqa: E712
+            AdminNotificationConfig.notify_on_errors == True,  # noqa: E712
+        )
+    )
+    configs = result.scalars().all()
+
+    if not configs:
+        return 0
+
+    sent = 0
+    for config in configs:
+        try:
+            success = await _send_apprise(str(config.apprise_url), title, body)
+            if success:
+                sent += 1
+        except Exception as exc:
+            logger.warning(
+                "Failed to send admin notification via channel %s: %s",
+                config.id,
+                exc,
+            )
+
+    return sent
+
+
+async def test_notification(apprise_url: str) -> tuple[bool, str]:
+    """
+    Send a test notification to the given Apprise URL.
+
+    Returns:
+        (success, message) tuple
+    """
+    try:
+        success = await _send_apprise(
+            apprise_url,
+            title="InboxRescue: Test Notification",
+            body="This is a test notification from InboxRescue. Your notification channel is configured correctly!",
+        )
+        if success:
+            return True, "Test notification sent successfully"
+        return False, "Notification delivery failed (check your Apprise URL)"
+    except Exception as exc:
+        return False, f"Error sending test notification: {exc}"
+
+
+async def _send_apprise(url: str, title: str, body: str) -> bool:
+    """Internal helper – create an Apprise instance, load the URL, and notify."""
+    ap = apprise.Apprise()
+    if not ap.add(url):
+        logger.warning("Apprise could not parse URL: %s", url[:60])
+        return False
+
+    result = await ap.async_notify(title=title, body=body)
+    return bool(result)