feat(db,shared): add overbookingAcknowledged field and conflict check types

- Assignment.overbookingAcknowledged Boolean @default(false) — audit trail
  for intentional overbookings
- CreateAssignmentBaseSchema gets allowOverbooking?: boolean flag so callers
  can explicitly opt in to overbooking
- Export AllocationConflictCheckResult, AllocationConflictDay,
  AllocationVacationOverlap types from @capakraken/shared for use in the
  new conflict-check API procedure and AllocationModal

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

packages/db/prisma/schema.prisma

@@ -1334,6 +1334,7 @@ model Assignment {
   dailyCostCents          Int
   status                  AllocationStatus  @default(PROPOSED)
   metadata                Json              @db.JsonB @default("{}")
+  overbookingAcknowledged Boolean           @default(false)
 
   demandRequirement   DemandRequirement? @relation(fields: [demandRequirementId], references: [id])
   resource            Resource           @relation(fields: [resourceId], references: [id])

packages/shared/src/schemas/allocation.schema.ts

@@ -43,6 +43,8 @@ export const CreateAssignmentBaseSchema = z.object({
   dailyCostCents: z.number().int().min(0).optional(),
   status: z.nativeEnum(AllocationStatus).default(AllocationStatus.PROPOSED),
   metadata: z.record(z.string(), z.unknown()).default({}),
+  /** When true the caller acknowledges the resource will be overbooked. */
+  allowOverbooking: z.boolean().optional().default(false),
 });
 
 /**
@@ -119,3 +121,34 @@ export type FillDemandRequirementInput = z.infer<typeof FillDemandRequirementSch
 export type FillOpenDemandByAllocationInput = z.infer<typeof FillOpenDemandByAllocationSchema>;
 export type ShiftProjectInput = z.infer<typeof ShiftProjectSchema>;
 export type UpdateAllocationHoursInput = z.infer<typeof UpdateAllocationHoursSchema>;
+
+// ─── Conflict check ───────────────────────────────────────────────────────────
+
+export interface AllocationConflictDay {
+  /** ISO date string (YYYY-MM-DD) */
+  date: string;
+  availableHours: number;
+  existingHours: number;
+  requestedHours: number;
+  overageHours: number;
+}
+
+export interface AllocationVacationOverlap {
+  startDate: string;
+  endDate: string;
+  /** VacationType enum value */
+  type: string;
+  isHalfDay: boolean;
+}
+
+export interface AllocationConflictCheckResult {
+  isOverbooking: boolean;
+  overbooking: {
+    conflictDays: AllocationConflictDay[];
+    totalConflictDays: number;
+    /** Highest single-day overage as a percentage above capacity */
+    maxOverbookPercent: number;
+  } | null;
+  vacationOverlap: AllocationVacationOverlap[];
+  hasVacationOverlap: boolean;
+}