Analytics: date-range filter + fix category endpoint Decimal crash

by-category and daily-spending accept start_date/end_date (range wins
over cycle); Analytics gets a two-month calendar range picker beside
the cycle selector. Fixes a latent float/Decimal TypeError that 500'd
spending_by_category whenever it had data — the category donut had
never rendered. Regression + range tests added.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/tests/test_analytics.py

@@ -0,0 +1,62 @@
+"""Analytics endpoints: category aggregation (Decimal regression) and the
+arbitrary date-range filters added for the Analytics range picker."""
+
+from datetime import datetime
+from decimal import Decimal
+
+from app.api.v1.endpoints.analytics import daily_spending, spending_by_category
+from app.models.models import Category, Transaction
+
+from tests.test_installments import make_cc_tx
+
+
+def seed(session):
+    cat = Category(name="Groceries")
+    session.add(cat)
+    session.commit()
+    make_cc_tx(session, merchant="A", amount="1000.50", reference="a1",
+               date=datetime(2026, 6, 2), category_id=cat.id)
+    make_cc_tx(session, merchant="B", amount="2000.00", reference="b1",
+               date=datetime(2026, 6, 20))
+    return cat
+
+
+class TestSpendingByCategory:
+    def test_decimal_sum_does_not_crash_and_percentages_add_up(self, session):
+        """Regression: Decimal grand total vs float row total raised
+        TypeError, 500ing the endpoint whenever it had data."""
+        seed(session)
+        rows = spending_by_category(session=session, _user="t")
+        assert {r.category_name for r in rows} == {"Groceries", "Uncategorized"}
+        assert sum(r.percentage for r in rows) == 100.0
+
+    def test_date_range_overrides_cycle(self, session):
+        seed(session)
+        rows = spending_by_category(
+            start_date="2026-06-01",
+            end_date="2026-06-10",
+            cycle_year=2026,
+            cycle_month=1,  # would match nothing; range must win
+            session=session,
+            _user="t",
+        )
+        assert len(rows) == 1
+        assert rows[0].category_name == "Groceries"
+        assert rows[0].total == 1000.50
+
+
+class TestDailySpending:
+    def test_date_range_filter(self, session):
+        seed(session)
+        rows = daily_spending(
+            start_date="2026-06-15", end_date="2026-07-01",
+            session=session, _user="t",
+        )
+        assert [r.date for r in rows] == ["2026-06-20"]
+        assert rows[0].total == 2000.00
+
+    def test_future_cuotas_excluded(self, session):
+        make_cc_tx(session, merchant="FUTURE", amount="500.00",
+                   reference="f1", date=datetime(2030, 1, 19))
+        rows = daily_spending(session=session, _user="t")
+        assert rows == []