feat: add new index, constraints on isQueued

Our queuing logic will be slow if we dont use an index.
isQueued is most of the time true, since emailoutbox table accounts for historical data and all sent emails have isQueued set to true.
So the index will actually be quite selective.
We can't just use the partial index on isQueued because its build on tenancyId, and the queuing query doesnt check tenancy id.

Author: nams1570

apps/backend/prisma/migrations/20260213004424_email_outbox_is_queued_index/migration.sql

@@ -0,0 +1,6 @@
+-- SPLIT_STATEMENT_SENTINEL
+-- SINGLE_STATEMENT_SENTINEL
+-- RUN_OUTSIDE_TRANSACTION_SENTINEL
+-- Index on isQueued for efficient queueReadyEmails() queries.
+-- Most emails have isQueued=TRUE (already processed), so filtering for FALSE is highly selective.
+CREATE INDEX CONCURRENTLY IF NOT EXISTS "EmailOutbox_isQueued_idx" ON /* SCHEMA_NAME_SENTINEL */."EmailOutbox" ("isQueued");

apps/backend/prisma/schema.prisma

@@ -833,7 +833,7 @@ model EmailOutbox {
   // The scheduled time of when the email should be added to the queue. Can be edited, but only if the email has not yet started sending. Doing so should set isQueued to false.
   scheduledAt DateTime
 
-  // The scheduled time of the email if it is in the future.
+  // Whether the email has been queued for sending. Once queued, this stays TRUE.
   isQueued Boolean @default(false)
 
   // A generated column that is equal to scheduledAt if isQueued is false, otherwise null. See the note above on EmailOutboxStatus.status for more details on dbgenerated values.
@@ -886,6 +886,7 @@ model EmailOutbox {
   @@index([tenancyId, finishedSendingAt(sort: Desc), scheduledAtIfNotYetQueued(sort: Desc), priority, id], map: "EmailOutbox_ordering_idx")
   @@index([tenancyId, simpleStatus], map: "EmailOutbox_simple_status_tenancy_idx")
   @@index([tenancyId, status], map: "EmailOutbox_status_tenancy_idx")
+  @@index([isQueued], map: "EmailOutbox_isQueued_idx")
 }
 
 model EmailOutboxProcessingMetadata {