compute_next_run parsed scheduled_time as "HH:MM" with int(parts[0]),
int(parts[1]) and no validation, so "9", "9am", "25:00", "9:" or ":30" raised
IndexError/ValueError. The POST /tasks create route passes the user/LLM-supplied
scheduled_time before its try block (and only validates the cron field), so a
bad value surfaced as an unhandled 500 rather than the clean 400 used for other
invalid fields — and the same crash could fire inside the scheduler loop when
recomputing next_run for an already-stored bad row.
Guard the parse and fail closed (warn + return None), matching the existing
invalid-cron handling in the same function.
Adds tests/test_scheduler_scheduled_time_validation.py — malformed values return
None (fail before with IndexError/ValueError), valid HH:MM still computes.
