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/api/src/router/assistant-tools.ts

@@ -4,7 +4,7 @@
  */
 
 import { prisma } from "@planarchy/db";
-import { calculateAllocation, countWorkingDays } from "@planarchy/engine/allocation";
+import { calculateAllocation, checkDuplicateAssignment, countWorkingDays } from "@planarchy/engine/allocation";
 import { computeBudgetStatus } from "@planarchy/engine";
 import type { PermissionKey } from "@planarchy/shared";
 import { parseTaskAction } from "@planarchy/shared";
@@ -2195,6 +2195,16 @@ const executors = {
     const startDate = new Date(params.startDate);
     const endDate = new Date(params.endDate);
 
+    // Check for overlapping duplicate assignments (same resource + project + overlapping dates)
+    const existingAssignments = await ctx.db.assignment.findMany({
+      where: { resourceId: resource.id, status: { not: "CANCELLED" } },
+      select: { id: true, resourceId: true, projectId: true, startDate: true, endDate: true, status: true },
+    });
+    const dupCheck = checkDuplicateAssignment(resource.id, project.id, startDate, endDate, existingAssignments);
+    if (dupCheck.isDuplicate) {
+      return { error: dupCheck.message + " Use update_allocation_status to modify the existing assignment." };
+    }
+
     // Check for existing CANCELLED allocation with same unique key — reactivate it
     const existing = await ctx.db.assignment.findUnique({
       where: {
@@ -3002,6 +3012,16 @@ const executors = {
     }
     if (!resource) return { error: `Resource not found: ${params.resourceId}` };
 
+    // Check for overlapping duplicate assignments (same resource + project + overlapping dates)
+    const existingAssignments = await ctx.db.assignment.findMany({
+      where: { resourceId: resource.id, status: { not: "CANCELLED" } },
+      select: { id: true, resourceId: true, projectId: true, startDate: true, endDate: true, status: true },
+    });
+    const dupCheck = checkDuplicateAssignment(resource.id, demand.project.id, demand.startDate, demand.endDate, existingAssignments);
+    if (dupCheck.isDuplicate) {
+      return { error: dupCheck.message + " Use update_allocation_status to modify the existing assignment." };
+    }
+
     const roleName = demand.roleEntity?.name ?? demand.role ?? null;
     const dailyCostCents = Math.round(resource.lcrCents * demand.hoursPerDay);
     const assignment = await ctx.db.assignment.create({