Add verified artifact evaluation

[jeewoo-lee]

Summary

• Add verified artifact task creation with hidden eval bundle metadata, verification attempts, artifact uploads, and local server_eval execution.
• Surface verified scores across run APIs, CLI output, UI leaderboard/run detail views, and documentation.
• Add focused server tests for verified task creation/submission plus fixture updates for task slug compatibility.

Test plan

• uv run pytest tests/server/ tests/cli/test_cmd_item.py -q --tb=no
• bash ci/run_all.sh (known failure: existing file-size check flags src/hive/server/main.py, src/hive/server/db.py, and src/hive/server/items.py over 500 lines)

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