Tasa Cero: InstallmentPlan model, cuota service, migration

BAC zero-interest financing: an anchor purchase splits into N monthly
cuota transactions (truncate-to-0.10 rule, last cuota absorbs rounding;
one cuota per 18th-cut billing cycle on the 19th). Pinned against real
Financiamientos data.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/services/installments.py

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+"""Tasa Cero (BAC zero-interest installment financing) plans.
+
+A qualifying credit-card purchase ("anchor") is billed by the bank as N equal
+monthly cuotas instead of one charge. Here the anchor transaction is flagged
+(`is_installment_anchor`) and excluded from budget aggregates, and N real
+cuota Transaction rows are generated so they flow through the existing
+billing-cycle logic — including future-dated cuotas, which intentionally show
+up in future months' budgets as committed spend.
+
+Verified BAC mechanics (real Financiamientos data, 2026-05/07):
+- cuota = total / N truncated to 0.10; the LAST cuota absorbs the remainder
+  (111,053.60/3 -> 37,017.80 x2 + 37,018.00).
+- Cuotas post one per 18th-cut billing cycle, on the 19th, except the first
+  one whose date varies per plan (purchase date or a later 19th) — hence
+  `first_installment_date` is stored per plan and user-editable.
+
+Known v1 gaps: paste-imported real "CUOTA:XX/YY" statement lines won't dedup
+against the synthetic cuotas; editing the anchor's amount does not recompute
+the plan (edit the plan instead).
+
+Cascades are handled explicitly here in Python: tests run on SQLite without
+FK enforcement, and transaction.installment_plan_id has no DB-level FK (it
+would be circular with installmentplan.anchor_transaction_id).
+"""
+
+import re
+from datetime import datetime
+from decimal import ROUND_DOWN, Decimal
+from typing import Optional
+
+from sqlmodel import Session, select
+
+from app.models.models import (
+    InstallmentPlan,
+    Transaction,
+    TransactionType,
+)
+from app.services.budget_projection import get_previous_cycle
+
+# BAC prefixes Tasa Cero voucher merchants with "CC " (observed with a double
+# space: "CC  CONSTRUPLAZA"). Non-prefixed plans (e.g. ALMACENES SIMAN) are
+# converted manually via POST /installment-plans/.
+TASA_CERO_RE = re.compile(r"^CC\s", re.IGNORECASE)
+
+DEFAULT_NUM_INSTALLMENTS = 3
+
+
+def is_tasa_cero_merchant(merchant: str) -> bool:
+    return bool(TASA_CERO_RE.match(merchant))
+
+
+def compute_installment_amounts(total: Decimal, n: int) -> list[Decimal]:
+    """Split `total` into n cuotas using BAC's rule: truncate to 0.10, last
+    cuota absorbs the remainder. Invariant: sum(result) == total."""
+    per = (total / n).quantize(Decimal("0.1"), rounding=ROUND_DOWN)
+    return [per] * (n - 1) + [total - per * (n - 1)]
+
+
+def _add_months(year: int, month: int, delta: int) -> tuple[int, int]:
+    idx = year * 12 + (month - 1) + delta
+    return idx // 12, idx % 12 + 1
+
+
+def compute_installment_dates(first: datetime, n: int) -> list[datetime]:
+    """Cuota 1 = `first` verbatim; cuotas 2..n land on the 19th of consecutive
+    billing cycles (cycle = [18th, next 18th), see get_cycle_range)."""
+    if first.day >= 18:
+        cycle_y, cycle_m = first.year, first.month
+    else:
+        cycle_y, cycle_m = get_previous_cycle(first.year, first.month)
+    dates = [first]
+    for i in range(1, n):
+        y, m = _add_months(cycle_y, cycle_m, i)
+        dates.append(datetime(y, m, 19))
+    return dates
+
+
+def _generate_cuotas(
+    session: Session, plan: InstallmentPlan, anchor: Transaction
+) -> None:
+    amounts = compute_installment_amounts(anchor.amount, plan.num_installments)
+    dates = compute_installment_dates(
+        plan.first_installment_date, plan.num_installments
+    )
+    merchant_label = " ".join(anchor.merchant.split())  # collapse "CC  X"
+    ref_base = anchor.reference or f"TC{anchor.id}"
+    n = plan.num_installments
+    for i, (amount, when) in enumerate(zip(amounts, dates), start=1):
+        session.add(
+            Transaction(
+                amount=amount,
+                currency=anchor.currency,
+                merchant=f"{merchant_label} CUOTA:{i:02d}/{n:02d}",
+                city=anchor.city,
+                date=when,
+                card_type=anchor.card_type,
+                card_last4=anchor.card_last4,
+                reference=f"{ref_base}-C{i:02d}",
+                transaction_type=TransactionType.COMPRA,
+                source=anchor.source,
+                bank=anchor.bank,
+                notes=f"Tasa Cero plan #{plan.id}",
+                category_id=anchor.category_id,
+                deferred_to_next_cycle=False,
+                installment_plan_id=plan.id,
+            )
+        )
+
+
+def _delete_cuotas(session: Session, plan: InstallmentPlan) -> None:
+    cuotas = session.exec(
+        select(Transaction).where(Transaction.installment_plan_id == plan.id)
+    ).all()
+    for cuota in cuotas:
+        session.delete(cuota)
+
+
+def create_plan(
+    session: Session,
+    anchor: Transaction,
+    num_installments: int = DEFAULT_NUM_INSTALLMENTS,
+    first_installment_date: Optional[datetime] = None,
+) -> InstallmentPlan:
+    """Convert `anchor` (must be committed, id set) into a Tasa Cero plan and
+    generate its cuota rows. Caller commits."""
+    amounts = compute_installment_amounts(anchor.amount, num_installments)
+    plan = InstallmentPlan(
+        anchor_transaction_id=anchor.id,
+        num_installments=num_installments,
+        first_installment_date=first_installment_date or anchor.date,
+        total_amount=anchor.amount,
+        installment_amount=amounts[0],
+        currency=anchor.currency,
+    )
+    session.add(plan)
+    session.flush()  # assign plan.id for the cuota FK/notes
+    anchor.is_installment_anchor = True
+    session.add(anchor)
+    _generate_cuotas(session, plan, anchor)
+    return plan
+
+
+def regenerate_cuotas(
+    session: Session,
+    plan: InstallmentPlan,
+    anchor: Transaction,
+    num_installments: Optional[int] = None,
+    first_installment_date: Optional[datetime] = None,
+) -> InstallmentPlan:
+    """Apply plan edits by delete-and-recreate of the cuota rows. Per-cuota
+    edits (e.g. recategorizations) are lost; category is re-inherited from
+    the anchor. Caller commits."""
+    if num_installments is not None:
+        plan.num_installments = num_installments
+    if first_installment_date is not None:
+        plan.first_installment_date = first_installment_date
+    plan.total_amount = anchor.amount
+    plan.installment_amount = compute_installment_amounts(
+        anchor.amount, plan.num_installments
+    )[0]
+    _delete_cuotas(session, plan)
+    _generate_cuotas(session, plan, anchor)
+    session.add(plan)
+    return plan
+
+
+def delete_plan(session: Session, plan: InstallmentPlan, anchor: Transaction) -> None:
+    """Unconvert: remove cuotas + plan, restore the anchor to a normal
+    transaction. Caller commits."""
+    _delete_cuotas(session, plan)
+    anchor.is_installment_anchor = False
+    session.add(anchor)
+    session.delete(plan)
+
+
+def teardown_plan_for_anchor(session: Session, anchor: Transaction) -> None:
+    """The anchor itself is being deleted: remove its plan and cuotas."""
+    if not anchor.is_installment_anchor:
+        return
+    plan = session.exec(
+        select(InstallmentPlan).where(
+            InstallmentPlan.anchor_transaction_id == anchor.id
+        )
+    ).first()
+    if plan:
+        _delete_cuotas(session, plan)
+        session.delete(plan)