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>

apps/web/src/components/allocations/AllocationModal.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { useState, useEffect, useRef } from "react";
+import { useState, useEffect, useRef, useMemo } from "react";
 import { useFocusTrap } from "~/hooks/useFocusTrap.js";
 import { useInvalidatePlanningViews } from "~/hooks/useInvalidatePlanningViews.js";
 import { AllocationStatus } from "@planarchy/shared";
@@ -73,6 +73,36 @@ export function AllocationModal({ allocation, onClose, onSuccess }: AllocationMo
     { staleTime: 60_000 },
   );
 
+  // Fetch existing allocations for the selected resource+project to detect overlaps
+  const shouldCheckOverlap = !isDemandEntry && !!resourceId && !!projectId;
+  const { data: existingAllocations } = trpc.allocation.listView.useQuery(
+    { projectId, resourceId },
+    { enabled: shouldCheckOverlap, staleTime: 30_000 },
+  );
+
+  const overlapWarning = useMemo(() => {
+    if (!shouldCheckOverlap || !existingAllocations || !startDate || !endDate) return null;
+    const formStart = new Date(startDate);
+    const formEnd = new Date(endDate);
+    if (isNaN(formStart.getTime()) || isNaN(formEnd.getTime())) return null;
+
+    const allocList = (existingAllocations as { allocations?: Array<{ id: string; resourceId?: string | null; startDate: string | Date; endDate: string | Date }> }).allocations ?? [];
+    for (const existing of allocList) {
+      // Skip the allocation being edited
+      if (isEditing && allocation && existing.id === allocation.id) continue;
+      // Only check assignments for this resource
+      if (existing.resourceId !== resourceId) continue;
+      const exStart = new Date(existing.startDate);
+      const exEnd = new Date(existing.endDate);
+      // Check date overlap
+      if (formStart <= exEnd && formEnd >= exStart) {
+        const fmt = (d: Date) => d.toISOString().slice(0, 10);
+        return `This resource is already assigned to this project from ${fmt(exStart)} to ${fmt(exEnd)}. Consider updating the existing assignment instead.`;
+      }
+    }
+    return null;
+  }, [shouldCheckOverlap, existingAllocations, startDate, endDate, isEditing, allocation, resourceId]);
+
   const invalidatePlanningViews = useInvalidatePlanningViews();
 
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
@@ -473,6 +503,13 @@ export function AllocationModal({ allocation, onClose, onSuccess }: AllocationMo
             </div>
           )}
 
+          {/* Overlap warning */}
+          {overlapWarning && (
+            <div className="rounded-lg border border-amber-300 bg-amber-50 px-3 py-2 text-sm text-amber-800 dark:border-amber-700 dark:bg-amber-950/30 dark:text-amber-300">
+              {"\u26A0"} {overlapWarning}
+            </div>
+          )}
+
           {/* Footer */}
           <div className="flex items-center justify-end gap-3 pt-2">
             <button