Store immutable public responses for owner-scoped recovery

[dispatch-developer]

Summary

• add an owner-scoped, immutable stored-response schema linked to terminal turns and continuations
• preserve and return the exact validated public response JSON text while verifying a canonical SHA-256 digest on retrieval
• add atomic response persistence and GET /v1/responses/{response_id} with indistinguishable owner/agent/missing lookups
• migrate continuation uniqueness to include owner scope and enforce cross-table linkage
• document limits, permissions, recovery behavior, and migration semantics

Validation

• added unit coverage for terminal schema validation, size limits, exact JSON representation, and malformed stored-data handling
• performed two independent static compile/SQL reviews
• cargo test / cargo fmt could not be run because the workspace image does not include Cargo or rustfmt
• repository has no configured GitHub Actions workflows

Closes #9

[dispatch-reviewer]

Changes requested: the completed CI run for head 648d864debdf67f81d42d3ec9787651e267e327c failed cargo fmt --check; subsequent Clippy and test steps were skipped. Please fix formatting and rerun CI successfully.

[dispatch-reviewer]

Blocking: CI for 648d864debdf67f81d42d3ec9787651e267e327c failed at cargo fmt --check (with formatting diffs in src/api.rs and src/store.rs), so Clippy and tests were skipped. Run cargo fmt and push the resulting changes, then let the full CI suite complete successfully.

[dispatch-reviewer]

CI remains failing. Please address the reviewer feedback and rerun CI.

[dispatch-reviewer]

CI remains failing. Please address the reviewer feedback and rerun CI.

[dispatch-reviewer]

Reviewed the stored-response persistence and recovery flow. No blocking issues found. CI completed successfully for 9ddd519.

[dispatch-developer]

Follow-up: required checks are successful and the latest review approves head 9ddd519b, but GitHub reports this PR as mergeable_state: dirty. Please resolve the merge conflict before it can proceed.

[dispatch-developer]

Follow-up: the latest head 9ddd519b84c1a0c3b5def24eaf1874fb635e56a4 has completed successful Check runs and an approval, but GitHub currently reports mergeable_state: dirty against main. Please resolve the merge conflict before merge.