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/models/models.py

@@ -169,6 +169,17 @@ class Transaction(TransactionBase, table=True):
     id: Optional[int] = Field(default=None, primary_key=True)
     created_at: datetime = Field(default_factory=utcnow)
     category: Optional[Category] = Relationship(back_populates="transactions")
+    # Tasa Cero: set on generated cuota rows. Plain int (no DB FK) to avoid a
+    # circular transaction<->installmentplan dependency; cascades are handled
+    # explicitly in app.services.installments (SQLite tests don't enforce FKs
+    # anyway). Table-class only so TransactionCreate/n8n payloads can't set it.
+    installment_plan_id: Optional[int] = Field(default=None, index=True)
+    # Tasa Cero: True on the original purchase once converted to a plan; the
+    # anchor stays visible in listings but is excluded from budget/analytics
+    # aggregates (its generated cuotas are counted instead).
+    is_installment_anchor: bool = Field(
+        default=False, sa_column_kwargs={"server_default": "false"}
+    )
 
 
 class TransactionCreate(TransactionBase):
@@ -179,6 +190,8 @@ class TransactionRead(TransactionBase):
     id: int
     created_at: datetime
     category: Optional[CategoryRead] = None
+    installment_plan_id: Optional[int] = None
+    is_installment_anchor: bool = False
 
 
 class TransactionUpdate(SQLModel):
@@ -194,6 +207,39 @@ class TransactionUpdate(SQLModel):
     deferred_to_next_cycle: Optional[bool] = None
 
 
+# --- Installment Plan (Tasa Cero) ---
+
+
+class InstallmentPlanBase(SQLModel):
+    num_installments: int
+    first_installment_date: datetime
+    total_amount: Money = Field(max_digits=15, decimal_places=2)
+    # All cuotas but the last; the last absorbs the rounding remainder.
+    installment_amount: Money = Field(max_digits=15, decimal_places=2)
+    currency: Currency = Currency.CRC
+    notes: Optional[str] = None
+
+
+class InstallmentPlan(InstallmentPlanBase, table=True):
+    id: Optional[int] = Field(default=None, primary_key=True)
+    anchor_transaction_id: int = Field(
+        foreign_key="transaction.id", ondelete="CASCADE", unique=True, index=True
+    )
+    created_at: datetime = Field(default_factory=utcnow)
+
+
+class InstallmentPlanCreate(SQLModel):
+    transaction_id: int
+    num_installments: int = 3
+    first_installment_date: Optional[datetime] = None  # default: anchor.date
+
+
+class InstallmentPlanUpdate(SQLModel):
+    num_installments: Optional[int] = None
+    first_installment_date: Optional[datetime] = None
+    notes: Optional[str] = None
+
+
 # --- Exchange Rate ---