fix: address remaining CodeRabbit review issues in backend

[Copilot]

Four issues flagged by CodeRabbit remained unresolved: JWT encoding mismatch between Program.cs and AuthController, missing GitHubId unique index in the initial migration, and misleading success response when a user record is missing during quest submission.

Changes

• Program.cs — Changed Encoding.ASCII → Encoding.UTF8 for JWT signing key bytes to match AuthController.cs. Mismatched encodings cause silent token validation failures for any key with non-ASCII characters.

• InitialCreate.cs migration — Changed GitHubId column type from nvarchar(max) to nvarchar(450) and added IX_Users_GitHubId unique index directly in the initial schema. GitHubId is the stable external identity; GitHubUsername is mutable and insufficient as the sole uniqueness constraint.

• AddUniqueConstraints.cs migration — Removed now-redundant AlterColumn for GitHubId and CreateIndex/DropIndex for IX_Users_GitHubId since these are handled in InitialCreate.

• IssuesController.cs — SubmitQuest — Return 404 immediately when the user record is not found, rather than silently skipping XP/streak updates and returning a misleading "XP Awarded!" with totalXp: null.

var user = await _context.Users.FindAsync(userId);
if (user == null)
    return NotFound("User account not found.");

user.ExperiencePoints += issue.XPReward;
// ... streak logic ...
return Ok(new { message = $"{issue.XPReward} XP Awarded!", totalXp = user.ExperiencePoints });

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