MC-4875

Fix F1 predictor deadline/calendar automation after audit. Audit findings from Elmar request to double-check normal vs sprint weekend automation: High-risk gaps: 1. Calendar refresh writes qualifying_utc and race_utc, but not sprint_utc; all is_sprint races in config/races_2026.json are missing sprint_utc. 2. run_sprint_pick.py and run_weekly.sprint_window_open require sprint_utc, so sprint-only constructor/team submissions currently self-skip every sprint weekend. 3. Fixed f1-sprint-pick schedule at Saturday 06:00 UTC would miss China sprint even after sprint_utc is added, because China Sprint starts 03:00 UTC. 4. Fixed f1-prediction-saturday 12:00/19:00 UTC misses early qualifying races: Australia 05:00 UTC, China 07:00 UTC, Japan 06:00 UTC, Azerbaijan Friday 12:00 UTC, Las Vegas 04:00 UTC. 5. submission_window_open currently fails open when qualifying_utc is missing/unparseable; auto-submit should fail closed or alert loudly instead. 6. Sprint weekend full race submission currently passes/clicks sprint team too, despite task docs saying sprint pick is separate and should not be re-submitted by the full Saturday refresh. Scope: - Add sprint_utc (and preferably sprint_qualifying_utc) to calendar refresh from FastF1 sprint session fields. - Make submission_window_open fail closed on missing/unparseable qualifying_utc for auto-submit. - Separate sprint constructor/team submission from full race submission: full race submit should not click sprint team after sprint window is closed; preferably sprint-only task owns sprint field. - Make scheduler strategy deadline-aware enough not to miss early qualifying/sprint cutoffs. If dynamic scheduler support is too large, add safe frequent gated windows and document why they are safe. - Keep normal weekend behavior: non-sprint submissions must not require sprint fields. - Add regression tests covering sprint_utc presence, early qualifying/sprint deadlines, non-sprint no-sprint-field behavior, and sprint/full separation. - Do not run live Superbru submit except existing tests/dry-run/no-submit paths. Acceptance: - Tests pass. - Scheduler/task docs match code behavior for normal vs sprint weekends. - Changes committed and pushed. - Final comment includes exact normal/sprint behavior and next race timing evidence.