PLU-615: cascade flow_connections (#1332)

## TL;DR
Deleting a connection from `connections` does not cascade to
`flow_connections`.

## What changed
* Delete from `flow_connections` when a connection is deleted.
* Add index on `connection_id` in `flow_connections`
* Show `Not connected` for a collaborator when the connection is
deleted.


## Tests
1. Delete a connection from a Pipe that has collaborators
2. Verify that the connection has been soft deleted in both
`connections` and `flow_connections`
3. Should show 'Not connected' on the UI

## Deploy Notes
- [ ] Run migration `20251124023536_add_index_on_flow_connections.ts`

Author: kevinkim-ogp

packages/backend/src/db/migrations/20251124023536_add_index_on_flow_connections.ts

@@ -0,0 +1,13 @@
+import { Knex } from 'knex'
+
+export async function up(knex: Knex): Promise<void> {
+  await knex.schema.table('flow_connections', (table) => {
+    table.index(['connection_id'])
+  })
+}
+
+export async function down(knex: Knex): Promise<void> {
+  await knex.schema.table('flow_connections', (table) => {
+    table.dropIndex(['connection_id'])
+  })
+}

packages/backend/src/graphql/mutations/delete-connection.ts

@@ -1,19 +1,28 @@
+import FlowConnections from '@/models/flow-connections'
+
 import type { MutationResolvers } from '../__generated__/types.generated'
 
 const deleteConnection: MutationResolvers['deleteConnection'] = async (
   _parent,
   params,
   context,
 ) => {
-  await context.currentUser
-    .$relatedQuery('connections')
-    .delete()
-    .findOne({
-      id: params.input.id,
-    })
-    .throwIfNotFound()
+  return await FlowConnections.transaction(async (trx) => {
+    await context.currentUser
+      .$relatedQuery('connections', trx)
+      .delete()
+      .findOne({
+        id: params.input.id,
+      })
+      .throwIfNotFound()
+
+    // also delete the connection from the flow_connections table for flows with collaborators
+    await FlowConnections.query(trx)
+      .delete()
+      .where({ connection_id: params.input.id })
 
-  return true
+    return true
+  })
 }
 
 export default deleteConnection