perf(api): eliminate N+1 queries, add query guards and missing indexes

- Notification fan-out: replace sequential for loops with Promise.all (allocation-effects, notification-broadcast, create-notification)
- Public holiday batch: group resources by location combo, resolve holidays once per group, replace per-holiday delete/findFirst/create with 3 batched queries (~18K → ~5 queries)
- Add take guards to unbounded findMany calls (resource-analytics: 5000, resource-marketplace: 2000, resource-capacity: 1000, chargeability-report: 2000)
- auto-staffing: add select with only needed fields + take: 5000
- schema.prisma: add 5 missing indexes (ManagementLevel.groupId, Blueprint.isActive/target, Comment.parentId, Vacation.requestedById, Resource.managementLevelGroupId)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/db/prisma/schema.prisma

@@ -751,6 +751,7 @@ model ManagementLevel {
   createdAt DateTime @default(now())
   updatedAt DateTime @updatedAt
 
+  @@index([groupId])
   @@map("management_levels")
 }
 
@@ -778,6 +779,8 @@ model Blueprint {
   createdAt       DateTime        @default(now())
   updatedAt       DateTime        @updatedAt
 
+  @@index([isActive])
+  @@index([target])
   @@map("blueprints")
 }
 
@@ -904,6 +907,7 @@ model Resource {
   @@index([countryId])
   @@index([orgUnitId])
   @@index([resourceType])
+  @@index([managementLevelGroupId])
   @@map("resources")
 }
 
@@ -1347,6 +1351,8 @@ model Assignment {
   @@index([status])
   @@index([resourceId, status, startDate])
   @@index([projectId, startDate, endDate])
+  @@index([status, startDate, endDate])
+  @@index([projectId, status, startDate, endDate])
   @@map("assignments")
 }
 
@@ -1386,6 +1392,7 @@ model Vacation {
   @@index([startDate, endDate])
   @@index([status])
   @@index([resourceId, status, startDate, endDate])
+  @@index([requestedById])
   @@map("vacations")
 }
 
@@ -1649,6 +1656,7 @@ model Comment {
 
   @@index([entityType, entityId])
   @@index([authorId])
+  @@index([parentId])
   @@map("comments")
 }