CSV export for transactions with download buttons (ARCH-18)

GET /api/v1/transactions/export streams a CSV (optional source/type/
date filters), cookie-authenticated so window.open downloads work.
Buttons on the Budget transactions tab (all) and Salarios (SALARY
only). Quoting and category resolution covered by tests.

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

backend/app/services/csv_export.py

@@ -0,0 +1,57 @@
+"""CSV export of transactions (review ARCH-18 / wishlist #4)."""
+
+import csv
+import io
+from datetime import datetime
+from typing import Optional
+
+from sqlmodel import Session, col, select
+
+from app.models.models import Transaction, TransactionSource, TransactionType
+
+COLUMNS = [
+    "id", "date", "merchant", "city", "amount", "currency",
+    "transaction_type", "source", "bank", "category", "notes", "reference",
+]
+
+
+def build_transactions_csv(
+    session: Session,
+    *,
+    source: Optional[TransactionSource] = None,
+    transaction_type: Optional[TransactionType] = None,
+    start_date: Optional[datetime] = None,
+    end_date: Optional[datetime] = None,
+) -> str:
+    q = select(Transaction)
+    if source:
+        q = q.where(Transaction.source == source)
+    if transaction_type:
+        q = q.where(Transaction.transaction_type == transaction_type)
+    if start_date:
+        q = q.where(Transaction.date >= start_date)
+    if end_date:
+        q = q.where(Transaction.date < end_date)
+    q = q.order_by(col(Transaction.date).desc(), col(Transaction.id).desc())
+
+    buf = io.StringIO()
+    writer = csv.writer(buf)
+    writer.writerow(COLUMNS)
+    for tx in session.exec(q).all():
+        writer.writerow(
+            [
+                tx.id,
+                tx.date.isoformat(),
+                tx.merchant,
+                tx.city or "",
+                f"{tx.amount:.2f}",
+                tx.currency.value,
+                tx.transaction_type.value,
+                tx.source.value,
+                tx.bank.value,
+                tx.category.name if tx.category else "",
+                tx.notes or "",
+                tx.reference or "",
+            ]
+        )
+    return buf.getvalue()