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/api/v1/endpoints/transactions.py

@@ -2,6 +2,7 @@ from datetime import datetime
 from typing import Optional
 
 from fastapi import APIRouter, Depends, HTTPException, Query
+from fastapi.responses import Response as FastAPIResponse
 from pydantic import BaseModel
 from sqlalchemy import and_, or_
 from sqlmodel import Session, col, func, select
@@ -21,6 +22,7 @@ from app.models.models import (
 )
 
 from app.services.budget_projection import get_cycle_range, get_previous_cycle
+from app.services.csv_export import build_transactions_csv
 from app.services.exchange_rate import get_converted_amount_expr
 
 router = APIRouter(prefix="/transactions", tags=["transactions"])
@@ -97,6 +99,33 @@ def list_transactions(
     return session.exec(query).all()
 
 
+@router.get("/export")
+def export_transactions_csv(
+    source: Optional[TransactionSource] = None,
+    transaction_type: Optional[TransactionType] = None,
+    start_date: Optional[str] = None,
+    end_date: Optional[str] = None,
+    session: Session = Depends(get_session),
+    _user: str = Depends(get_current_user),
+):
+    """Download transactions as CSV (filters optional). Cookie-authenticated,
+    so a plain <a href> download works from the SPA."""
+    csv_text = build_transactions_csv(
+        session,
+        source=source,
+        transaction_type=transaction_type,
+        start_date=datetime.fromisoformat(start_date) if start_date else None,
+        end_date=datetime.fromisoformat(end_date) if end_date else None,
+    )
+    return FastAPIResponse(
+        content=csv_text,
+        media_type="text/csv; charset=utf-8",
+        headers={
+            "Content-Disposition": 'attachment; filename="wealthysmart-transactions.csv"'
+        },
+    )
+
+
 @router.get("/cycles", response_model=list[BillingCycle])
 def list_billing_cycles(
     session: Session = Depends(get_session),