feat(discord): reschedule recurring occurrences via scheduled-event exceptions

[LizAinslie]

Stacked on #28 (feat/discord-two-way-sync); merge that first. Base of this PR is the feat/discord-two-way-sync branch.

What / why

Dragging a single occurrence of a recurring imported Discord event now writes a scheduled-event exception (override) locally and pushes it to Discord, instead of rescheduling the whole series.

Notable files

• server/src/main/resources/db/migration/V20__discord_event_exceptions.sql
• server/src/main/kotlin/dev/kolektiv/kalendee/external/store/ExternalEventStore.kt (+ Postgres impl), oauth/discord/DiscordEventMapping.kt
• server/src/main/kotlin/dev/kolektiv/kalendee/oauth/discord/DiscordPushService.kt, oauth/providers/DiscordApi.kt
• server/pack/src/lib/components/WeekGrid.svelte, docs/external-calendars.md

Test evidence

• ./gradlew :server:test :core:jvmTest --no-daemon — server 342 tests, 0 failures; core 56 tests, 0 failures
• pnpm --dir server/pack typecheck — clean

CI note

CI requires the Kord fork artifact dev.kord:kord-rest:0.18.1-kalendee.2 published to repo.yuri.capital maven-releases. Fork: https://github.com/KolektivComputer/kord, branch kalendee/recurrence.

Closes #23