fix(cache): expire stale evaluation fallbacks

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Summary

• Add a bounded freshness policy for miner evaluation cache fallback.
• Track issue-discovery cache freshness separately from the PR-side cache entry.
• Prevent expired issue-discovery aggregates from being restored through either direct issue fallback or PR cache fallback.

What changed

• Added EVALUATION_CACHE_MAX_AGE_SECONDS with a six-hour fallback window.
• Added freshness checks to MinerEvaluationCache.get() and a dedicated get_issue_discovery() accessor.
• Preserved issue-discovery fields across OSS stores only while their own refresh timestamp is fresh.
• Added cache unit tests and issue-discovery regression coverage for expired fallback behavior.

Why

Cache fallback is useful for short-lived upstream failures, but old evaluation data should not remain reward-active indefinitely. The PR-side and issue-discovery cache fields refresh at different points in the validator round, so both need explicit freshness handling.

Validation

• uv run pytest tests/validator/test_miner_evaluation_cache.py tests/validator/issue_discovery/test_scan.py tests/validator/test_github_identity_fallback.py tests/validator/oss_contributions/mirror/test_routing.py tests/validator/oss_contributions/mirror/test_classes_integration.py -q
• uv run ruff check gittensor/classes.py gittensor/constants.py gittensor/validator/issue_discovery/scan.py tests/validator/test_miner_evaluation_cache.py tests/validator/issue_discovery/test_scan.py
• uv run ruff format --check gittensor/classes.py gittensor/constants.py gittensor/validator/issue_discovery/scan.py tests/validator/test_miner_evaluation_cache.py tests/validator/issue_discovery/test_scan.py
• uv run pyright gittensor/classes.py gittensor/constants.py gittensor/validator/issue_discovery/scan.py tests/validator/test_miner_evaluation_cache.py tests/validator/issue_discovery/test_scan.py
• git diff --check
• Codex Security source-to-sink review: transient PR fetch failures and mirror issue fetch failures now require fresh matching cache state; expired issue fields are stripped from PR fallback results.
• CodeRabbit CLI review: blocked by free-tier rate limit after authentication; no CodeRabbit findings were available for this PR.