chore(pm): vendor the Aramesh/PM platform under pm/ (squash of Aramesh/PM @ 91f3271)

The PM panel (PmPanelPage -> iframe /pm/) needs the project-management
platform served at /pm/ on the ERP domain. Source: org repo Aramesh/PM,
branch feature/project-management-platform. Update via:
git subtree pull --prefix=pm aramesh-pm feature/project-management-platform --squash

pm/backend/tests/test_date_service.py

@@ -0,0 +1,48 @@
+"""Unit tests for unified DateService."""
+from datetime import date
+import pytest
+
+from app.utils.date_service import date_service
+
+
+def test_date_service_parse_iso():
+    d = date_service.parse("2026-09-16")
+    assert d == date(2026, 9, 16)
+
+
+def test_date_service_parse_jalali():
+    # 1405/06/25 -> 2026-09-16
+    d = date_service.parse("1405/06/25")
+    assert d == date(2026, 9, 16)
+
+
+def test_date_service_to_jalali():
+    d = date(2026, 9, 16)
+    assert date_service.to_jalali(d) == "1405/06/25"
+
+
+def test_date_service_duration_calculation():
+    d1 = date(2026, 9, 10)
+    d2 = date(2026, 9, 15)
+    # gap between the dates: the end date is not counted
+    assert date_service.calculate_duration(d1, d2) == 5
+    assert date_service.calculate_duration(d1, d1) == 0
+    assert date_service.calculate_duration(None, d2) == 0
+    # inverted range is not a duration
+    assert date_service.calculate_duration(d2, d1) == 0
+
+
+def test_date_service_compute_end_date():
+    d1 = date(2026, 9, 10)
+    # 0 days means start and end are the same day
+    assert date_service.compute_end_date(d1, 0) == d1
+    # 5 days is the gap, so the end date is 5 days later
+    assert date_service.compute_end_date(d1, 5) == date(2026, 9, 15)
+
+
+def test_duration_and_end_date_are_inverses():
+    """The pair must round-trip, otherwise saved values drift on reload."""
+    d1 = date(2026, 9, 10)
+    for gap in (0, 1, 4, 5, 30):
+        end = date_service.compute_end_date(d1, gap)
+        assert date_service.calculate_duration(d1, end) == gap