feat(api,application): add checkConflicts query and soften overbooking block

- New allocation.checkConflicts managerProcedure: returns per-day overbooking
  breakdown (availableHours, existingHours, requestedHours, overageHours,
  maxOverbookPercent) plus vacation overlap list for the requested period.
  Read-only — used by AllocationModal for pre-submission warnings.
- createAssignment(): replace the hard >5-day overbooking block with a soft
  CONFLICT error. When allowOverbooking: true is passed the assignment is
  created and overbookingAcknowledged is set to true on the record.
- allowOverbooking field added to CreateAssignmentBaseSchema (optional)

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

packages/application/src/use-cases/allocation/create-assignment.ts

@@ -143,10 +143,10 @@ export async function createAssignment(
     availabilityWindows,
   );
 
-  if (!availabilityResult.valid && availabilityResult.totalConflictDays > 5) {
+  if (!availabilityResult.valid && !input.allowOverbooking) {
     throw new TRPCError({
-      code: "BAD_REQUEST",
-      message: `Resource has availability conflicts on ${availabilityResult.totalConflictDays} days`,
+      code: "CONFLICT",
+      message: `Resource is overbooked on ${availabilityResult.totalConflictDays} day(s). Pass allowOverbooking: true to proceed.`,
     });
   }
 
@@ -179,6 +179,7 @@ export async function createAssignment(
       dailyCostCents: input.dailyCostCents ?? calculation.dailyCostCents,
       status: input.status,
       metadata: input.metadata as unknown as Prisma.InputJsonValue,
+      overbookingAcknowledged: input.allowOverbooking === true && !availabilityResult.valid,
     },
     include: ASSIGNMENT_RELATIONS_INCLUDE,
   });
@@ -194,3 +195,55 @@ export async function createAssignment(
 
   return assignment;
 }
+
+/**
+ * Creates an assignment record without running availability or duplicate checks.
+ * Use ONLY for fragment operations (splitting/carving an existing allocation)
+ * where the resource's total load does not change.
+ */
+export async function createAssignmentFragment(
+  db: DbClient,
+  input: {
+    demandRequirementId?: string | null | undefined;
+    resourceId: string;
+    projectId: string;
+    startDate: Date;
+    endDate: Date;
+    hoursPerDay: number;
+    percentage: number;
+    role?: string | null | undefined;
+    roleId?: string | null | undefined;
+    dailyCostCents: number;
+    status: string;
+    metadata: unknown;
+  },
+): Promise<AssignmentWithRelations> {
+  const assignment = await db.assignment.create({
+    data: {
+      demandRequirementId: input.demandRequirementId ?? null,
+      resourceId: input.resourceId,
+      projectId: input.projectId,
+      startDate: input.startDate,
+      endDate: input.endDate,
+      hoursPerDay: input.hoursPerDay,
+      percentage: input.percentage,
+      role: input.role ?? null,
+      roleId: input.roleId ?? null,
+      dailyCostCents: input.dailyCostCents,
+      status: input.status as import("@capakraken/db").AllocationStatus,
+      metadata: input.metadata as import("@capakraken/db").Prisma.InputJsonValue,
+    },
+    include: ASSIGNMENT_RELATIONS_INCLUDE,
+  });
+
+  await db.auditLog.create({
+    data: {
+      entityType: "Assignment",
+      entityId: assignment.id,
+      action: "CREATE",
+      changes: { after: assignment } as unknown as import("@capakraken/db").Prisma.InputJsonValue,
+    },
+  });
+
+  return assignment;
+}