Backend — Send Structured Transaction Updates via WebSocket

[jbair06]

Description:
Update WebSocket transaction messages to send structured and complete transaction data (or transaction nodes/IDs, if necessary) instead of generic “transaction_action” events. Ensure only relevant clients receive updates.

Tasks:

• Replace { action: "transaction_action" } with { action: "transaction_updated", transactions: [ {...} ] }.
• Include full transaction payloads (with safe data fields only).
• Target delivery to relevant channels (e.g., per user/org/key) instead of broadcasting globally.
• Add validation tests for correct payload shape and routing.

Acceptance criteria:

• WebSocket messages always include the updated transaction(s).
• Message routing matches intended recipients only.
• No sensitive fields appear in payloads.
• Backward compatibility is handled (if needed).