Preserve user context for parsed challenge assignments

[raphink]

What changed

• Added a user-scoped challenge assignment query helper.
• Updated GetUserChallenge(..., WithParsedAssignmentVariables()) to fetch assignment text with userID + challengeID instead of falling back to the public challenge query.
• Added tests covering parsed assignment variables and user-scoped assignment fetching.

Why

The parsed assignment path needs user context for runtime variable interpolation. Calling GetChallengeWithAssignment() from GetUserChallenge() dropped that context, so parseAssignmentVariables was effectively moot for user sessions.

Validation

• go test ./...