Allow deleting claims, creating standalone payments, and add a donations tab

Claims could previously only be cancelled (stornieren), which blocks once a
payment is allocated. Add a hard delete that releases any linked
payments/credits back to being unallocated instead of destroying them.

Payments could only be created from within a claim, forcing immediate
allocation. Add a bare payment creation flow in the Zahlungen tab so
incoming transfers can be logged first and allocated later.

Add a per-member Spenden tab (donations, backed by a new donations list on
ContributionData) so amounts paid beyond the membership fee can be tracked
and existing free payments allocated to them.

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

src/ccma/domain/contributions.py

@@ -18,6 +18,13 @@ CLAIM_STATUS_LABELS = {
     "cancelled": "STORNIERT",
 }
 
+DONATION_STATUS_LABELS = {
+    "open": "OFFEN",
+    "partially_allocated": "TEILWEISE ZUGEORDNET",
+    "allocated": "ZUGEORDNET",
+    "overallocated": "ÜBERZUGEORDNET",
+}
+
 
 def decimal_value(value: Any, field_name: str = "Betrag") -> Decimal:
     text = str(value).strip().replace(",", ".")
@@ -109,6 +116,38 @@ def claim_balance(data: ContributionData, claim: dict[str, Any]) -> Decimal:
     return (claim_total(claim) - allocated_total(data, str(claim.get("claim_id", "")))).quantize(CENT)
 
 
+def donation_allocated_total(data: ContributionData, donation_id: str) -> Decimal:
+    return sum(
+        (
+            decimal_value(allocation.get("amount", "0"))
+            for allocation in data.allocations
+            if str(allocation.get("donation_id", "")) == donation_id
+        ),
+        Decimal("0"),
+    )
+
+
+def donation_amount(donation: dict[str, Any]) -> Decimal:
+    return decimal_value(donation.get("amount", "0"))
+
+
+def donation_balance(data: ContributionData, donation: dict[str, Any]) -> Decimal:
+    donation_id = str(donation.get("donation_id", ""))
+    return (donation_amount(donation) - donation_allocated_total(data, donation_id)).quantize(CENT)
+
+
+def donation_status(data: ContributionData, donation: dict[str, Any]) -> str:
+    balance = donation_balance(data, donation)
+    allocated = donation_allocated_total(data, str(donation.get("donation_id", "")))
+    if balance < 0:
+        return "overallocated"
+    if balance == 0:
+        return "allocated"
+    if allocated > 0:
+        return "partially_allocated"
+    return "open"
+
+
 def claim_status(data: ContributionData, claim: dict[str, Any], *, today: date | None = None) -> str:
     if str(claim.get("status", "")) == "cancelled":
         return "cancelled"