feat(enterprise): add scoped storage repository

[Gracker]

Summary

• close §0.3.1 by selecting SQLite WAL for enterprise v1 storage and documenting the decision in ADR-0001
• add a workspace-scoped enterprise repository abstraction that derives scope from RequestContext and always applies tenant/workspace filters
• guard scoped repository criteria and updates so callers cannot override tenant_id/workspace_id or mutate identity columns
• add SQLite WAL connection tests and repository scope isolation tests to backend test:core
• mark README §0.3.1 complete and register the ADR in §0.9

Validation

• PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npx jest --runInBand --forceExit src/services/__tests__/enterpriseDb.test.ts src/services/__tests__/enterpriseRepository.test.ts in backend
• PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run test:core in backend
• git diff --check
• PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run verify:pr