feat: Parser-Basis, Bank-Erkennung, Saldo-/Duplikat-Pruefung

- ParsedTransaction und ParsedStatement Datenklassen
- parse_german_amount(): German format number parsing (1.234,56 -> Decimal)
- parse_german_date(): DD.MM.YYYY date parsing with optional year
- detect_bank(): Recognize VR/HVB/DKB from text markers
- balance_difference(): Validate statement balance consistency
- dedup_hash(): SHA256-based transaction deduplication

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

finance/app/parsers/base.py

@@ -0,0 +1,46 @@
+import re
+from dataclasses import dataclass, field
+from datetime import date
+from decimal import Decimal
+
+
+class ParserError(Exception):
+    pass
+
+
+@dataclass(frozen=True)
+class ParsedTransaction:
+    booking_date: date
+    value_date: date | None
+    amount: Decimal
+    purpose: str
+    counterparty: str
+
+
+@dataclass
+class ParsedStatement:
+    bank: str
+    iban: str | None
+    period_start: date | None
+    period_end: date | None
+    opening_balance: Decimal | None
+    closing_balance: Decimal | None
+    transactions: list[ParsedTransaction] = field(default_factory=list)
+
+
+def parse_german_amount(s: str) -> Decimal:
+    s = s.strip().replace(" ", "").replace(" ", "")
+    neg = s.endswith("-") or s.startswith("-") or s.endswith("S")
+    s = s.strip("+-SH")
+    value = Decimal(s.replace(".", "").replace(",", "."))
+    return -value if neg else value
+
+
+def parse_german_date(s: str, default_year: int | None = None) -> date:
+    m = re.fullmatch(r"(\d{2})\.(\d{2})\.(\d{4})?", s.strip())
+    if not m:
+        raise ParserError(f"Kein Datum: {s!r}")
+    year = int(m.group(3)) if m.group(3) else default_year
+    if year is None:
+        raise ParserError(f"Jahr fehlt: {s!r}")
+    return date(year, int(m.group(2)), int(m.group(1)))