OIC persistence: schema, migrations, and append-only claim history #221

Context

Accepted knowledge needs a durable store that never silently overwrites history. Depends on #220.

Behavior to implement

• SQL schema and migrations for every domain type in OIC domain model: claims, evidence, entities, scopes, and two timelines #220, running on SQLite and Postgres.
• Append-only claim versions: revising a claim writes a new version row with who, when, why, and which investigation caused it. The previous version stays readable.
• Repositories (read/write services) that the application layer uses. Nothing outside them touches tables.
• Indexes for the queries the API needs: by scope, entity, claim type, status, and valid time.

Acceptance criteria

• Migrations apply on an empty SQLite file and on Postgres.
• Revising a claim twice leaves three readable versions, and "what was true on date D" returns the right one.
• New evidence never deletes older evidence.