feat(notifications): add Test button for ntfy and RSS with inline result

- POST /api/notifications/test/ntfy — sends a real push using current form
  values (not saved settings) so auth can be verified before saving; returns
  status + HTTP detail on success or error message on failure
- POST /api/notifications/test/rss — confirms the feed token exists and
  returns event count; no bill FK required
- NtfyTestRequest + NotificationTestResult schemas added
- Frontend: Test button next to Save on both ntfy and RSS sections; result
  shown inline as a green/red pill; uses current form state for ntfy so
  the user can test before committing

All future notification types should follow the same test-before-save pattern.

Authored-By: Jack Levy

backend/app/schemas/schemas.py

@@ -28,6 +28,20 @@ class NotificationSettingsUpdate(BaseModel):
     ntfy_enabled: Optional[bool] = None
     rss_enabled: Optional[bool] = None
 
+
+class NtfyTestRequest(BaseModel):
+    ntfy_topic_url: str
+    ntfy_auth_method: str = "none"
+    ntfy_token: str = ""
+    ntfy_username: str = ""
+    ntfy_password: str = ""
+
+
+class NotificationTestResult(BaseModel):
+    status: str  # "ok" | "error"
+    detail: str
+    event_count: Optional[int] = None  # RSS only
+
 T = TypeVar("T")