feat: add saved public playlists

[Priveetee]

Summary

• add persistent hideContinueWatching support to GET /settings and PUT /settings
• add protected saved public playlist endpoints for listing, saving and deleting playlist references
• validate saved public playlists through extraction and keep duplicate saves idempotent per user/url
• document the saved public playlist contract in OpenAPI

Notes

• saved public playlists store playlist references and metadata, not copied videos
• frontend can open saved entries through the existing public playlist extraction flow
• existing settings rows default hideContinueWatching to false

Runtime verification

• GET /settings includes hideContinueWatching
• PUT /settings persists hideContinueWatching=true then false
• saved playlist smoke verified: initial [], POST /saved-playlists returns 201, GET /saved-playlists returns the saved reference, DELETE /saved-playlists/{id} returns 204, final list returns []

Tests

• JAVA_HOME=/usr/lib/jvm/java-21-openjdk ./gradlew test
• JAVA_HOME=/usr/lib/jvm/java-21-openjdk ./gradlew shadowJar
• JAVA_HOME=/usr/lib/jvm/java-21-openjdk ./gradlew validateOpenApi