Add individually agreed membership fees (contribution overrides)

Members are sometimes given a fee that deviates from the regular
schedule -- e.g. a reduced rate for students -- for a specific period.
Add a per-member "Beitrag" tab where such deviations can be recorded
with a month-granular date range (Ab/Bis), a mandatory reason, and
either a fixed annual amount or a percentage discount off whichever
base rate is in effect at the time.

Data model: ContributionData gets a contribution_overrides list, each
entry validated (month format, Bis >= Ab, non-overlapping ranges per
member, reason required) and CRUD'd through the repository
(record/update/delete/get_contribution_override), consistent with how
donations already work.

Integration: contribution_claims.py now computes each membership-fee
claim's amount month by month instead of a single rate for the whole
billing period, picking up whichever override (if any) covers each
individual month. That handles an override starting or ending mid
period correctly (e.g. a semiannual payer whose discount begins in
March) without changing behavior for members without overrides.
Already-created claims are never recalculated retroactively, matching
how changes to the global contribution rates already behave.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_dates.py

@@ -7,9 +7,11 @@ from ccma.domain.dates import (
     age_label,
     calculate_age,
     format_date_for_display,
+    format_month_for_display,
     normalize_date_input,
     parse_date_input,
     parse_iso_date,
+    parse_month_input,
     validate_birth_date,
     validate_member_dates,
 )
@@ -68,6 +70,25 @@ def test_member_dates_must_be_chronological() -> None:
         )
 
 
+def test_month_input_accepts_german_and_iso_formats() -> None:
+    assert parse_month_input("03.2026", "Ab") == "2026-03"
+    assert parse_month_input("2026-03", "Ab") == "2026-03"
+    assert parse_month_input("", "Ab") == ""
+    with pytest.raises(DateValidationError, match="erforderlich"):
+        parse_month_input("", "Ab", allow_empty=False)
+    with pytest.raises(DateValidationError):
+        parse_month_input("13.2026", "Ab")
+    with pytest.raises(DateValidationError):
+        parse_month_input("2026-03-15", "Ab")
+
+
+def test_month_display_uses_system_pattern(monkeypatch) -> None:
+    monkeypatch.setattr("ccma.domain.dates.system_date_pattern", lambda: "%d.%m.%Y")
+    assert format_month_for_display("2026-03") == "03.2026"
+    monkeypatch.setattr("ccma.domain.dates.system_date_pattern", lambda: "%Y-%m-%d")
+    assert format_month_for_display("2026-03") == "2026-03"
+
+
 def test_age_calculation_and_label() -> None:
     today = date(2026, 6, 21)
     assert calculate_age(date(2000, 6, 21), today) == 26