feat(garmin): mark a wrapper 404 with not_found in the error response

garminconnect's own GarminConnectNotFoundError already exists
specifically for this (its docstring: "so callers can now catch a
missing resource specifically, e.g. deleting an already-deleted
workout"), raised by connectapi() for any real HTTP 404. dispatch()
now surfaces that distinction as an extra not_found: true field
alongside the existing error string, so internal/garmin/client.go
(next commit) can tell a definitive 404 apart from a transient
failure.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/internal/garmin/pyscript/wrapper.py

@@ -9,7 +9,7 @@ import sys
 import threading
 import traceback
 
-from garminconnect import Garmin
+from garminconnect import Garmin, GarminConnectNotFoundError
 
 TOKENSTORE = os.path.expanduser(os.environ.get("GARMIN_TOKENSTORE", "~/.garmin"))
 
@@ -208,7 +208,15 @@ def dispatch(req):
     except Exception as exc:
         _debug(f"{req.get('cmd')} raised {type(exc).__name__}: {exc}")
         _debug(traceback.format_exc())
-        return {"id": req.get("id"), "error": str(exc)}
+        resp = {"id": req.get("id"), "error": str(exc)}
+        # A 404 (e.g. get_workout_by_id for a workout deleted on Garmin's
+        # side after being linked to an activity) is definitive, not a
+        # transient failure worth retrying forever -- marked specifically so
+        # internal/garmin/client.go can tell the two apart (see
+        # docs/superpowers/specs/2026-07-27-workout-not-found-design.md).
+        if isinstance(exc, GarminConnectNotFoundError):
+            resp["not_found"] = True
+        return resp
 
 
 def main():