Fix My UI holiday and summer scheduling

[ProfessorPolymorphic]

Summary

• keep My UI on its Monday cadence during university holidays
• add the June–July My UI Monday schedule
• preserve the noon-Wednesday submission deadline
• keep university closure blackouts scoped to The Daily Register
• migrate existing schedule and seeded blackout rows safely

Root cause

Seeded university holidays were stored as global blackout rows, so the scheduling service applied them to both newsletters. My UI was also configured to shift holiday editions and had no active June–July schedule. Because production seeding preserves existing schedule rows, correcting the JSON alone would not repair deployed databases.

Impact

My UI accepts Monday publication dates such as Labor Day and continues publishing Mondays in June and July. The Daily Register continues to skip university closure dates. Existing environments are corrected automatically by the Alembic migration.

Validation

• backend: 145 tests passed
• frontend: 41 tests passed
• backend Ruff checks passed
• frontend ESLint passed
• production TypeScript/Vite build passed
• Alembic upgrade and schema-drift checks passed
• migration verified against a database containing the old global blackout and stale schedule rows

Closes #201