feat: add assessment attempt and ranking flow

[Mindev27]

What changed

• Added server-managed assessment attempts with start, active-attempt resume, and attempt-based submission APIs.
• Enforced the 15-minute answer-input lock and calculated elapsed time from server timestamps.
• Added first-submission-only official ranking ordered by score descending and elapsed time ascending, including shared ranks for exact ties.
• Added reattempt support with rankEligible=false for non-first submissions.
• Added fixed 1–5 multi-select validation for multiple-choice answers and direct-input normalization for short answers.
• Extended assessment registration responses with assessmentId for the admin PDF/QR workflow.
• Added the additive production DB migration and runbook.

Why

Printed MathRank assessments need a QR-driven mobile flow where students start an attempt, wait 15 minutes before entering answers, submit for server grading, and see their official rank for that assessment.

Database

The additive migration was applied to the production mathrank database after creating the snapshot database-2-pre-assessment-attempt-20260729. Postflight checks confirmed the new column, table, constraints, and indexes. Existing assessments retain a zero-second delay and the attempt table was empty after migration.

Verification

• Focused assessment attempt and ranking tests
• Assessment API response test
• Assessment API Checkstyle
• Monolith bootJar
• git diff --check
• Live Swagger recheck: production is still v1.0.0-beta.21 with 53 paths / 73 operations and does not yet expose the new attempt APIs

Deployment note

Merging this PR does not deploy production by itself. The repository deployment workflow runs on a new v* tag, so create the next release tag only after review and merge.

[gemini-code-assist]

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