Apply grading PR review feedback: PII, locking, trigger cleanup, perf, docs

[Copilot]

Addresses all review comments from the grading PR (#4). Changes span correctness, privacy, performance, and documentation.

Privacy

• AUTO_GRADE_STATE keyed by submission.userId instead of respondent email — avoids persisting PII in Script Properties accessible to all editors.
• Submission is fetched before the "first attempt" check so userId is available early.

Correctness & Reliability

• LockService.getScriptLock() wraps the state read-modify-write in handleAutoGradeFormSubmit — prevents concurrent trigger executions from racing and corrupting "first attempt" tracking.
• Response sheet selection changed from .find() (first match) to .filter() + last element — ensures the most recently created "Form Responses" tab is used when multiple tabs exist.
• Alert wording fixed: "Skipped (state not TURNED_IN)" → "Skipped (state neither TURNED_IN nor RETURNED)".

Trigger Quota

• ensureAutoGradeTriggerForForm now deletes orphaned handleAutoGradeFormSubmit triggers whose form IDs are no longer in AUTO_GRADE_TARGETS, preventing accumulation against Apps Script trigger quotas.

Performance

• getLatestResponseScoresByEmail reads only the header row first to locate Email/Score columns, then fetches just those two columns — avoids loading the entire sheet into memory for large response sets.

Docs

• README and Questions.sheet-format.md: updated MCQ/MSQ → MCQ-only for answer image URL feature, consistent with MSQ removal.

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