feat: prevent duplicate resource-project assignments

Engine (packages/engine):
- New checkDuplicateAssignment() pure function: detects same resource
  assigned to same project with overlapping dates
- 15 unit tests covering: overlap, no-overlap, cancelled, self-exclude,
  string dates, PROPOSED status

Application layer (packages/application):
- createAssignment: throws CONFLICT before DB write if duplicate found
- fillDemandRequirement: same check before entering transaction

AI Assistant (packages/api/router/assistant-tools.ts):
- create_allocation: checks before creating, returns helpful error message
- fill_demand: same check using demand's projectId

UI (apps/web):
- AllocationModal: amber warning when resource already assigned to
  selected project with overlapping dates (non-blocking)

Database cleanup:
- Found and merged 1 duplicate: Wong Wong on Porsche Taycan Sport Film
  (2 overlapping PROPOSED assignments merged into 1)

Regression: 298 engine tests pass (283 + 15 new). TypeScript clean.

Co-Authored-By: claude-flow <ruv@ruv.net>

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

@@ -1,4 +1,4 @@
-import { calculateAllocation, validateAvailability } from "@planarchy/engine";
+import { calculateAllocation, validateAvailability, checkDuplicateAssignment } from "@planarchy/engine";
 import type { PrismaClient, Prisma } from "@planarchy/db";
 import {
   type Allocation,
@@ -105,6 +105,28 @@ export async function createAssignment(
     },
   );
 
+  const duplicateResult = checkDuplicateAssignment(
+    input.resourceId,
+    input.projectId,
+    input.startDate,
+    input.endDate,
+    existingBookings.map(b => ({
+      id: b.id,
+      resourceId: b.resourceId ?? "",
+      projectId: b.projectId,
+      startDate: b.startDate,
+      endDate: b.endDate,
+      status: b.status,
+    })),
+  );
+
+  if (duplicateResult.isDuplicate) {
+    throw new TRPCError({
+      code: "CONFLICT",
+      message: duplicateResult.message ?? "Resource is already assigned to this project with overlapping dates",
+    });
+  }
+
   const availability = resource.availability as unknown as WeekdayAvailability;
   const availabilityWindows = existingBookings.map((booking) => ({
     startDate: booking.startDate,

packages/application/src/use-cases/allocation/fill-demand-requirement.ts

@@ -1,8 +1,10 @@
 import type { PrismaClient } from "@planarchy/db";
 import { AllocationStatus, type FillDemandRequirementInput } from "@planarchy/shared";
+import { checkDuplicateAssignment } from "@planarchy/engine";
 import { TRPCError } from "@trpc/server";
 import { type AssignmentWithRelations } from "./create-assignment.js";
 import { fillDemandRequirementWithLegacySync } from "./fill-demand-requirement-with-legacy-sync.js";
+import { listAssignmentBookings } from "./list-assignment-bookings.js";
 
 export interface FillDemandRequirementResult {
   assignment: AssignmentWithRelations;
@@ -54,5 +56,34 @@ export async function fillDemandRequirement(
     });
   }
 
+  const existingBookings = await listAssignmentBookings(
+    db as unknown as Parameters<typeof listAssignmentBookings>[0],
+    {
+      resourceIds: [input.resourceId],
+    },
+  );
+
+  const duplicateResult = checkDuplicateAssignment(
+    input.resourceId,
+    demandRequirement.projectId,
+    demandRequirement.startDate,
+    demandRequirement.endDate,
+    existingBookings.map(b => ({
+      id: b.id,
+      resourceId: b.resourceId ?? "",
+      projectId: b.projectId,
+      startDate: b.startDate,
+      endDate: b.endDate,
+      status: b.status,
+    })),
+  );
+
+  if (duplicateResult.isDuplicate) {
+    throw new TRPCError({
+      code: "CONFLICT",
+      message: duplicateResult.message ?? "Resource is already assigned to this project with overlapping dates",
+    });
+  }
+
   return fillDemandRequirementWithLegacySync(db, demandRequirement, input);
 }