fix(cron): treat non-dict origin as missing instead of crashing tick

``_resolve_origin`` called ``origin.get('platform')`` on whatever
``job.get('origin')`` returned. The leading ``if not origin: return None``
short-circuited the falsy cases (None, empty dict, "") but a non-empty
string passed that guard and then crashed with
``AttributeError: 'str' object has no attribute 'get'`` on every fire
attempt. Observed in the wild after a migration script tagged jobs with
free-form provenance strings (e.g.
``"combined-digest-replaces-x-and-y-20260503"``).

``mark_job_run`` did record ``last_status: error,
last_error: "'str' object has no attribute 'get'"`` once, but the next
tick re-loaded the same poisoned origin and crashed identically. The
job stayed enabled, fired every tick, and accumulated cascading errors
in the log until ``origin`` was patched manually.

Replace the falsy guard with ``isinstance(origin, dict)``. Non-dict
origins (string, int, list, tuple, float — anything that survived a
hand-edit, JSON-script write, or migration) are now treated the same
as a missing origin: the job continues with ``deliver`` falling back
through its normal home-channel path instead of crashing the scheduler
loop.

Test parametrises the non-dict shapes that can appear in jobs.json
through external writers and asserts ``_resolve_origin`` returns None
for each.

Note: this fix scope is the non-dict-``origin`` crash only. The
``next_run_at: null`` recurring-job recovery (the second sub-bug in
#18722) is independently addressed by the in-flight #18825, which
extends the never-silently-disable defense from #16265 to
``get_due_jobs()`` — that approach is well-aligned with the existing
recovery pattern and ships fine without a competing change here.

Fixes #18722 (non-dict origin crash; recurring-job recovery covered by #18825)

cron/scheduler.py

@@ -123,9 +123,19 @@ _LOCK_FILE = _LOCK_DIR / ".tick.lock"
 
 
 def _resolve_origin(job: dict) -> Optional[dict]:
-    """Extract origin info from a job, preserving any extra routing metadata."""
+    """Extract origin info from a job, preserving any extra routing metadata.
+
+    Treats non-dict origins (free-form provenance strings, ints, lists from
+    migration scripts or hand-edited jobs.json) as missing instead of
+    crashing with ``AttributeError`` on ``origin.get(...)``. Without this
+    guard, a job tagged with e.g. ``"combined-digest-replaces-x-and-y"``
+    crashed every fire attempt with
+    ``'str' object has no attribute 'get'`` — ``mark_job_run`` recorded the
+    failure, but the next tick re-loaded the same poisoned origin and
+    crashed identically until the field was patched manually (#18722).
+    """
     origin = job.get("origin")
-    if not origin:
+    if not isinstance(origin, dict):
         return None
     platform = origin.get("platform")
     chat_id = origin.get("chat_id")