Durable local data model and migrations for runs/findings/cases

[Shreyas582]

Objective: persist and evolve investigation data safely over time.

Scope:

• Define schema for runs, findings, cases, and evidence bundles.
• Add migration framework and compatibility policy.
• Add backup/export/import commands.

Acceptance criteria:

• Upgrades preserve existing local data without manual intervention.

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Dependency and Order

• Sequence slot: v1.0.0 step 3/4
• Blocked by: Implement local API server (wraithrun serve) with v1 endpoints #23
• Unblocks: Web UI MVP with live health panel and run triage #24
• Ready to start (dependencies met)

[Shreyas582]

Implemented in PR #90 (v1.0.0). SQLite WAL-mode store with schema_version migration table, runs/findings tables.