Add Discord Solve/First Blood Notifications, Nickname Synchronization, and 10-Character Name Limit

[yulmwu]

This PR adds Discord notifications for challenge solves. Messages are sent to the channel specified by DISCORD_ANNOUNCE_CHANNEL_ID, with First Blood solves receiving special emphasis.

Example:

🩸 First Blood! 청소년부_B — kim6wol0 solved member_directory test!
청소년부_A — KimJunYoung solved member_directory test

In addition, the Invite Bot now automatically sets a member's guild nickname to the format:

<Division>_<TeamName>_<Username>

If a user later updates their profile information, the bot automatically synchronizes the guild nickname to reflect the updated values.

Since Discord limits nicknames to 32 characters, the division name, team name, and username are each now limited to 10 characters to ensure the generated nickname remains within the limit.

For more details, please refer to the changes included in this PR.

Note

In a future update, different Discord roles and solve announcement channels will be configurable on a per-division basis.

[yulmwu]

It looks like the REST API documentation needs to be updated to reflect these changes.

[codecov]

Codecov Report

❌ Patch coverage is 78.57143% with 18 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/service/discord_service.go	64.58%	10 Missing and 7 partials ⚠️
internal/repo/submission_repo.go	93.33%	1 Missing ⚠️

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