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/engine/src/__tests__/duplicate-check.test.ts

@@ -0,0 +1,99 @@
+import { describe, it, expect } from "vitest";
+import { checkDuplicateAssignment, type ExistingAssignment } from "../allocation/duplicate-check.js";
+
+const base: ExistingAssignment = {
+  id: "a1",
+  resourceId: "r1",
+  projectId: "p1",
+  startDate: new Date("2026-03-01"),
+  endDate: new Date("2026-06-30"),
+  status: "CONFIRMED",
+};
+
+describe("checkDuplicateAssignment", () => {
+  it("returns no duplicate when no existing assignments", () => {
+    const result = checkDuplicateAssignment("r1", "p1", "2026-04-01", "2026-05-01", []);
+    expect(result.isDuplicate).toBe(false);
+  });
+
+  it("returns no duplicate for different project", () => {
+    const result = checkDuplicateAssignment("r1", "p2", "2026-04-01", "2026-05-01", [base]);
+    expect(result.isDuplicate).toBe(false);
+  });
+
+  it("returns no duplicate for different resource", () => {
+    const result = checkDuplicateAssignment("r2", "p1", "2026-04-01", "2026-05-01", [base]);
+    expect(result.isDuplicate).toBe(false);
+  });
+
+  it("returns no duplicate when dates don't overlap (before)", () => {
+    const result = checkDuplicateAssignment("r1", "p1", "2026-01-01", "2026-02-28", [base]);
+    expect(result.isDuplicate).toBe(false);
+  });
+
+  it("returns no duplicate when dates don't overlap (after)", () => {
+    const result = checkDuplicateAssignment("r1", "p1", "2026-07-01", "2026-08-01", [base]);
+    expect(result.isDuplicate).toBe(false);
+  });
+
+  it("detects duplicate with full overlap", () => {
+    const result = checkDuplicateAssignment("r1", "p1", "2026-03-15", "2026-05-15", [base]);
+    expect(result.isDuplicate).toBe(true);
+    expect(result.conflictingAssignment?.id).toBe("a1");
+    expect(result.message).toContain("already assigned");
+  });
+
+  it("detects duplicate with partial overlap (start before, end during)", () => {
+    const result = checkDuplicateAssignment("r1", "p1", "2026-02-15", "2026-04-01", [base]);
+    expect(result.isDuplicate).toBe(true);
+  });
+
+  it("detects duplicate with partial overlap (start during, end after)", () => {
+    const result = checkDuplicateAssignment("r1", "p1", "2026-05-01", "2026-07-15", [base]);
+    expect(result.isDuplicate).toBe(true);
+  });
+
+  it("detects duplicate on exact same dates", () => {
+    const result = checkDuplicateAssignment("r1", "p1", "2026-03-01", "2026-06-30", [base]);
+    expect(result.isDuplicate).toBe(true);
+  });
+
+  it("ignores CANCELLED assignments", () => {
+    const cancelled = { ...base, status: "CANCELLED" };
+    const result = checkDuplicateAssignment("r1", "p1", "2026-04-01", "2026-05-01", [cancelled]);
+    expect(result.isDuplicate).toBe(false);
+  });
+
+  it("ignores DRAFT assignments", () => {
+    const draft = { ...base, status: "DRAFT" };
+    const result = checkDuplicateAssignment("r1", "p1", "2026-04-01", "2026-05-01", [draft]);
+    expect(result.isDuplicate).toBe(false);
+  });
+
+  it("excludes self by ID (for updates)", () => {
+    const result = checkDuplicateAssignment("r1", "p1", "2026-04-01", "2026-05-01", [base], "a1");
+    expect(result.isDuplicate).toBe(false);
+  });
+
+  it("does not exclude other assignments when excludeId is set", () => {
+    const other: ExistingAssignment = { ...base, id: "a2" };
+    const result = checkDuplicateAssignment("r1", "p1", "2026-04-01", "2026-05-01", [other], "a1");
+    expect(result.isDuplicate).toBe(true);
+  });
+
+  it("works with string dates", () => {
+    const strAssignment: ExistingAssignment = {
+      ...base,
+      startDate: "2026-03-01",
+      endDate: "2026-06-30",
+    };
+    const result = checkDuplicateAssignment("r1", "p1", "2026-04-01", "2026-05-01", [strAssignment]);
+    expect(result.isDuplicate).toBe(true);
+  });
+
+  it("checks PROPOSED status as active", () => {
+    const proposed = { ...base, status: "PROPOSED" };
+    const result = checkDuplicateAssignment("r1", "p1", "2026-04-01", "2026-05-01", [proposed]);
+    expect(result.isDuplicate).toBe(true);
+  });
+});