feat: metadata drift detector between backend and on-chain records (#…

[codewithzubair07]

Fixes #882

Detects when off-chain backend metadata (ProjectView/ProjectMilestone) falls out of sync with on-chain identifiers or status.

• MetadataDriftDetector (src/metadata_drift_detector.py) recomputes canonical project/milestone state from the immutable ContractEvent log and diffs it against the mutable ProjectView/ProjectMilestone materialized views. Strictly read-only with respect to source data.
• New MetadataDriftFinding table + PostgresService persistence methods (save/get/mark-reviewed), following the existing RoundAnomalySignal review pattern.
• scripts/detect_metadata_drift.py for manual execution (--project-id, --no-persist, --json).
• Registered as a 6-hourly scheduled job in src/scheduler.py.
• tests/test_metadata_drift_detector.py: 8 tests covering no-drift baseline, status/amount/milestone drift, missing records, non-mutation of source data, multi-project runs, and persistence of findings.
• METADATA_DRIFT_DETECTOR.md mapping the implementation to each acceptance criterion.

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