Use Kord for the Discord REST client

[LizAinslie]

Replace the hand-rolled Discord REST client with Kord (soft fork), dev.kord:kord-rest:0.18.1-kalendee.2.

What

• DiscordApi.kt is rewritten on the Kord REST client instead of manual HTTP/JSON plumbing.
• The fork artifact is resolved from mavenLocal() during development (gradle/libs.versions.toml, settings.gradle.kts, server/build.gradle.kts).
• Discord tests are adapted to the Kord-backed client (DiscordApiTest, DiscordActionsTest, DiscordImportServiceTest, DiscordProviderTest, OAuthFoundationTest).

Test evidence

• :server:test — 319 tests, 0 failures
• :core:jvmTest — 56 tests, 0 failures

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.