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>

src/ccma/rules/scripts/contribution_claims.py

@@ -2,6 +2,7 @@ import calendar
 from datetime import date, timedelta
 from decimal import ROUND_HALF_UP, Decimal
 
+from ccma.domain.contributions import contribution_override_covers_month
 from ccma.domain.dates import DateValidationError, parse_iso_date
 from ccma.rules.api import RuleContext, create_claim
 from ccma.rules.scripts._shared import CONTRIBUTION_STATUSES
@@ -124,7 +125,7 @@ def _membership_claims(
         periods = [("annual", 1, 12, _due_date(year, rule.get("annual_due"), "01-31"))]
 
     actions = []
-    monthly_amount = annual_amount / Decimal(12)
+    overrides = context.contributions.contribution_overrides
     for suffix, first_month, last_month, regular_due in periods:
         # The entry year is intentionally billed from the entry month onward,
         # even when retroactive claims create old membership-fee claims.
@@ -132,7 +133,17 @@ def _membership_claims(
         months = max(0, last_month - charged_from + 1)
         if months == 0:
             continue
-        amount = (monthly_amount * months).quantize(CENT, rounding=ROUND_HALF_UP)
+        # Blended month by month rather than a single rate for the whole period, so a
+        # contribution override that starts or ends mid-period (e.g. a student discount
+        # beginning in March for a member who pays semiannually) is billed correctly.
+        month_amounts = [
+            _monthly_amount(context.repository_config, overrides, year, month)
+            for month in range(charged_from, last_month + 1)
+        ]
+        amount = sum((value for value, _override_id in month_amounts), Decimal("0")).quantize(
+            CENT, rounding=ROUND_HALF_UP
+        )
+        applied_overrides = sorted({override_id for _value, override_id in month_amounts if override_id})
         entry_year = started_at.year == year
         issue_date = regular_due - timedelta(days=issue_days)
         if not entry_year and context.today < issue_date:
@@ -168,6 +179,7 @@ def _membership_claims(
                         "annual_amount": _money(annual_amount),
                         "months": months,
                         "formula": "annual_amount * months / 12",
+                        "contribution_override_ids": applied_overrides,
                     },
                 },
             )
@@ -175,6 +187,28 @@ def _membership_claims(
     return actions
 
 
+def _monthly_amount(config, overrides, year: int, month: int) -> tuple[Decimal, str]:
+    """Returns the effective monthly rate for a single calendar month, and the
+    override's id if one applied (empty string otherwise)."""
+    rule = _rule_for(config, date(year, month, 1))
+    base_annual = Decimal(str(rule.get("annual_amount", "0"))) if rule else Decimal("0")
+    month_key = f"{year:04d}-{month:02d}"
+    override = next(
+        (item for item in overrides if contribution_override_covers_month(item, month_key)), None
+    )
+    if override is None:
+        return base_annual / Decimal(12), ""
+    kind = str(override.get("kind", ""))
+    value = Decimal(str(override.get("value", "0")))
+    if kind == "amount":
+        annual = value
+    elif kind == "percent":
+        annual = base_annual * (Decimal(100) - value) / Decimal(100)
+    else:
+        annual = base_annual
+    return annual / Decimal(12), str(override.get("override_id", ""))
+
+
 def _rule_for(config, target: date):
     selected = None
     for rule in config.get("contribution_rules", []):