feat: emission v1 + idempotency (Task 5)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

alembic/versions/30a80fe36910_fiscal_idempotency_keys.py

@@ -0,0 +1,48 @@
+"""fiscal_idempotency_keys (Task 5): backs the `Idempotency-Key` contract of
+`POST /v1/emissoes` (design spec decision #6) -- a separate table so the
+Task 3 `fiscal_documents` migration stays untouched. See
+`documents.models.FiscalIdempotencyKey`'s docstring for the two-layer
+idempotency pattern this table's UNIQUE constraint exists for.
+
+Revision ID: 30a80fe36910
+Revises: 523235d06bd5
+Create Date: 2026-07-22 00:00:00.000000
+
+"""
+from __future__ import annotations
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "30a80fe36910"
+down_revision: Union[str, Sequence[str], None] = "523235d06bd5"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "fiscal_idempotency_keys",
+        sa.Column("id", sa.UUID(), nullable=False),
+        sa.Column("product_id", sa.UUID(), nullable=False),
+        sa.Column("idempotency_key", sa.String(length=255), nullable=False),
+        sa.Column("document_id", sa.UUID(), nullable=False),
+        sa.Column(
+            "created_at", sa.DateTime(timezone=True), server_default=sa.text("now()"), nullable=False
+        ),
+        sa.Column(
+            "updated_at", sa.DateTime(timezone=True), server_default=sa.text("now()"), nullable=False
+        ),
+        sa.ForeignKeyConstraint(["product_id"], ["products.id"]),
+        sa.ForeignKeyConstraint(["document_id"], ["fiscal_documents.id"]),
+        sa.PrimaryKeyConstraint("id"),
+        sa.UniqueConstraint(
+            "product_id", "idempotency_key", name="uq_fiscal_idempotency_key_product_key"
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("fiscal_idempotency_keys")