Import payments per member from a GnuCash file

Adds a "Zahlungen importieren" button to the member Zahlungen tab that opens
a dedicated window: point it at a GnuCash file (plain or gzip-compressed
XML, defaults to the file configured in Optionen), pick one of its accounts,
and narrow the account's bookings down with a description-contains filter
and an optional date range.

Each matching booking gets a checkbox to mark it for import as a payment.
Before anything is ticked, bookings are cross-checked against this member's
existing payments by date + amount; a match is highlighted and its checkbox
is refused, so re-importing the same statement can't create a duplicate
payment. Only bookings with a positive amount on the selected account are
offered, since those are the ones that make sense as an incoming payment.

Parsing lives in ccma.services.gnucash_import, independent of the UI, and
is covered by tests against a synthetic GnuCash XML fixture (plain and
gzip-compressed) -- no gnucash/piecash dependency needed since the native
file format is just XML.

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

src/ccma/services/gnucash_import.py

@@ -0,0 +1,169 @@
+from __future__ import annotations
+
+import gzip
+from dataclasses import dataclass
+from datetime import date, datetime
+from decimal import Decimal, InvalidOperation
+from pathlib import Path
+from xml.etree import ElementTree as ET
+
+NS = {
+    "gnc": "http://www.gnucash.org/XML/gnc",
+    "act": "http://www.gnucash.org/XML/act",
+    "trn": "http://www.gnucash.org/XML/trn",
+    "split": "http://www.gnucash.org/XML/split",
+    "ts": "http://www.gnucash.org/XML/ts",
+}
+
+CENT = Decimal("0.01")
+
+
+class GnuCashImportError(RuntimeError):
+    pass
+
+
+@dataclass(frozen=True, slots=True)
+class GnuCashAccount:
+    guid: str
+    name: str
+    full_name: str
+    account_type: str
+
+
+@dataclass(frozen=True, slots=True)
+class GnuCashTransaction:
+    guid: str
+    date: date
+    description: str
+    memo: str
+    amount: Decimal
+
+
+def _read_root(path: Path) -> ET.Element:
+    try:
+        raw = path.read_bytes()
+    except OSError as exc:
+        raise GnuCashImportError(f"GnuCash-Datei konnte nicht gelesen werden: {exc}") from exc
+    if raw[:2] == b"\x1f\x8b":
+        try:
+            raw = gzip.decompress(raw)
+        except OSError as exc:
+            raise GnuCashImportError(f"GnuCash-Datei konnte nicht entpackt werden: {exc}") from exc
+    try:
+        return ET.fromstring(raw)
+    except ET.ParseError as exc:
+        raise GnuCashImportError(f"GnuCash-Datei ist kein gültiges XML: {exc}") from exc
+
+
+def _book(root: ET.Element) -> ET.Element:
+    book = root.find("gnc:book", NS)
+    if book is None:
+        raise GnuCashImportError("Keine GnuCash-Buchführung (gnc:book) in der Datei gefunden.")
+    return book
+
+
+def _text(element: ET.Element, path: str) -> str:
+    return (element.findtext(path, default="", namespaces=NS) or "").strip()
+
+
+def list_accounts(path: str | Path) -> list[GnuCashAccount]:
+    root = _read_root(Path(path))
+    book = _book(root)
+    by_guid: dict[str, ET.Element] = {}
+    parent_of: dict[str, str] = {}
+    for account_el in book.findall("gnc:account", NS):
+        guid = _text(account_el, "act:id")
+        if not guid:
+            continue
+        by_guid[guid] = account_el
+        parent_of[guid] = _text(account_el, "act:parent")
+    if not by_guid:
+        raise GnuCashImportError("Die Datei enthält keine Konten.")
+
+    def full_name(guid: str, seen: frozenset[str]) -> str:
+        account_el = by_guid.get(guid)
+        if account_el is None or guid in seen or _text(account_el, "act:type") == "ROOT":
+            return ""
+        name = _text(account_el, "act:name")
+        parent = parent_of.get(guid, "")
+        if not parent:
+            return name
+        parent_name = full_name(parent, seen | {guid})
+        return f"{parent_name}:{name}" if parent_name else name
+
+    accounts = []
+    for guid, account_el in by_guid.items():
+        account_type = _text(account_el, "act:type")
+        if account_type in {"ROOT", ""}:
+            continue
+        accounts.append(
+            GnuCashAccount(
+                guid=guid,
+                name=_text(account_el, "act:name"),
+                full_name=full_name(guid, frozenset()) or _text(account_el, "act:name"),
+                account_type=account_type,
+            )
+        )
+    accounts.sort(key=lambda item: item.full_name.casefold())
+    return accounts
+
+
+def _parse_fraction(text: str) -> Decimal | None:
+    text = text.strip()
+    if "/" not in text:
+        return None
+    numerator, _, denominator = text.partition("/")
+    try:
+        denom = Decimal(denominator)
+        if denom == 0:
+            return None
+        return (Decimal(numerator) / denom).quantize(CENT)
+    except InvalidOperation:
+        return None
+
+
+def _parse_posted_date(text: str) -> date | None:
+    text = text.strip()
+    if not text:
+        return None
+    for fmt in ("%Y-%m-%d %H:%M:%S %z", "%Y-%m-%d"):
+        try:
+            return datetime.strptime(text, fmt).date()
+        except ValueError:
+            continue
+    return None
+
+
+def list_transactions(path: str | Path, account_guid: str) -> list[GnuCashTransaction]:
+    """Return every booking that has a split on the given account, one row per
+    matching split (a transaction with two splits on the same account, e.g. a
+    transfer to itself, yields two rows -- rare in practice, harmless here)."""
+    root = _read_root(Path(path))
+    book = _book(root)
+    results: list[GnuCashTransaction] = []
+    for transaction_el in book.findall("gnc:transaction", NS):
+        guid = _text(transaction_el, "trn:id")
+        posted_date = _parse_posted_date(_text(transaction_el, "trn:date-posted/ts:date"))
+        if not guid or posted_date is None:
+            continue
+        description = _text(transaction_el, "trn:description")
+        splits_el = transaction_el.find("trn:splits", NS)
+        if splits_el is None:
+            continue
+        for split_el in splits_el.findall("trn:split", NS):
+            if _text(split_el, "split:account") != account_guid:
+                continue
+            amount = _parse_fraction(_text(split_el, "split:value"))
+            if amount is None:
+                continue
+            results.append(
+                GnuCashTransaction(
+                    guid=guid,
+                    date=posted_date,
+                    description=description,
+                    memo=_text(split_el, "split:memo"),
+                    amount=amount,
+                )
+            )
+    results.sort(key=lambda item: (item.date, item.description.casefold()))
+    return results