feat(enterprise): complete core storage schema

[Gracker]

Summary

• close §0.3.2 by extending the enterprise SQLite schema to cover every §10.2 core table
• add migration v4 for trace processor leases/holders, conversation turns, runtime snapshots, provider credentials, report artifacts, memory entries, skill registry entries, and tenant tombstones
• add owner-guard/replay/scope/tombstone indexes and composite tenant/workspace foreign-key coverage for the new required runtime/session/run chains
• expand enterprise schema tests to verify the §10.2 table set, indexes, migration versions, full insert chain, and cross-tenant reference rejection
• include the schema contract test in backend test:core and mark README §0.3.2 complete

Validation

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

[Gracker]

已通过 commit 67856ec (merge) + 023a84a (frontend rebuild) 合入 main，关闭此 PR。