fix(workflow): resume durable subscriptions without duplicate inference

[BerryUIKI]

Workflow submission is now idempotent and independent of its WebSocket subscribers. The backend persists the immutable request, node inputs/outcomes, provider job IDs and sequenced events. A reconnect only subscribes to the existing run after the last observed sequence. Disconnect does not cancel execution or submit a second inference request. Reusing a run ID with different content returns a conflict.

The canvas submits once over REST, resumes subscriptions and rejects obsolete/duplicate events. The unsupported application PING heartbeat is removed. Shutdown/restart records interrupted or uncertain work without paid resubmission. Existing raw graph WebSocket submissions remain compatible and idempotent when they supply a run ID.

Validation: 311 backend tests and 99 frontend tests passed; typecheck/lint passed (two pre-existing lint warnings). A real TestClient socket disconnect with a counted controllable runner asserts exactly one inference, resumed output, durable history and conflict rejection. Database reopen verifies replay; frontend tests assert cursor resume and stale-event isolation.

• Four small implementation/test commits
• Targets dev; all required CI must pass
• No live inference, secrets or generated runtime files

Fixes #116. Completes workflow durability for #120 alongside #172. Real remote-provider recovery remains unverified; unknown outcomes are never automatically retried.