Store PvP season startDate as TIMESTAMPTZ at rollover time

[gausie]

Summary

• Adds a migration to cast the PvpSeason.startDate column from DATE to TIMESTAMPTZ, shifting existing values by +3h30m to land at KoL rollover time
• Stores future seasons with the actual rollover timestamp (using now in the rollover handler) rather than midnight UTC
• Simplifies the calendar route — no offset arithmetic needed since timestamps are now correct by construction
• Fixes seasons appearing one day early in the calendar

Test plan

• Run migration — startDate for all existing seasons should read as 03:30:00+00 on the 1st of their month
• Navigate to a season start month in the calendar — ⚔️ should appear on the correct day (not the day before)