fix(core): soft-delete source connections with deleted_at

Migration 0003 adds source_connections.deleted_at. deleteConnection sets it; listing, lookups,
updates, the scheduler and the sync handler ignore deleted connections, so a paused
credential-less connection stays visible and can be resumed.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

packages/db/src/schema.ts

@@ -269,6 +269,8 @@ export interface SourceConnectionsTable {
   status: ConnectionStatus;
   last_run_at: string | null;
   created_at: string;
+  /** Set by `deleteConnection`; deleted connections are hidden, never scheduled and never synced. */
+  deleted_at: string | null;
 }
 
 export interface SourceRunsTable {