openspec: scan-alert-hardening change — different-finder re-alert + fingerprint 24h block

openspec/changes/scan-alert-hardening/specs/database/spec.md

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+## ADDED Requirements
+
+### Requirement: scans.fingerprint column
+The `scans` table SHALL include a nullable `fingerprint` text column, added idempotently so existing databases migrate cleanly.
+
+#### Scenario: Fresh database
+- **WHEN** `db/schema.sql` is applied to an empty database
+- **THEN** `scans.fingerprint` exists and is nullable
+
+#### Scenario: Existing database
+- **WHEN** `db/schema.sql` is re-applied to a database created before this column
+- **THEN** the column is added without error, nullable