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/domain/contributions.py

@@ -25,6 +25,11 @@ DONATION_STATUS_LABELS = {
     "overallocated": "ÜBERZUGEORDNET",
 }
 
+CONTRIBUTION_OVERRIDE_KIND_LABELS = {
+    "amount": "BETRAG",
+    "percent": "PROZENT",
+}
+
 
 def decimal_value(value: Any, field_name: str = "Betrag") -> Decimal:
     text = str(value).strip().replace(",", ".")
@@ -148,6 +153,26 @@ def donation_status(data: ContributionData, donation: dict[str, Any]) -> str:
     return "open"
 
 
+def contribution_override_covers_month(override: dict[str, Any], month: str) -> bool:
+    valid_from = str(override.get("valid_from", ""))
+    valid_until = str(override.get("valid_until", "") or "")
+    if valid_from and month < valid_from:
+        return False
+    if valid_until and month > valid_until:
+        return False
+    return True
+
+
+def contribution_override_ranges_overlap(
+    valid_from: str, valid_until: str, other_from: str, other_until: str
+) -> bool:
+    """Month ranges ('YYYY-MM', empty valid_until/other_until = open-ended) overlap
+    if each range starts no later than the other one ends."""
+    starts_before_other_ends = not other_until or valid_from <= other_until
+    other_starts_before_ends = not valid_until or other_from <= valid_until
+    return starts_before_other_ends and other_starts_before_ends
+
+
 def claim_status(data: ContributionData, claim: dict[str, Any], *, today: date | None = None) -> str:
     if str(claim.get("status", "")) == "cancelled":
         return "cancelled"