perf(db): cf_items.subject_uri に GIN 索引を追加

docs/spec/db-schema.md

@@ -99,6 +99,7 @@ last_change_date_time: TIMESTAMP NOT NULL
 UNIQUE(tenant_id, identifier)
 INDEX(tenant_id, cf_document_id, human_coding_scheme)  -- for upsert matching
 INDEX(cf_document_id, depth)  -- for the tree view's level detection (also covers `INDEX(cf_document_id)` alone)
+GIN INDEX(subject_uri jsonb_path_ops)  -- reverse lookup "items setting this subject": subject_uri @> '[{"identifier": ...}]'
 ```
 
 ### cf_association
@@ -348,6 +349,7 @@ last_change_date_time: TIMESTAMP NOT NULL
 UNIQUE(tenant_id, identifier)
 INDEX(tenant_id, cf_document_id, human_coding_scheme)  -- upsertマッチング用
 INDEX(cf_document_id, depth)  -- ツリービューLevel判定用（INDEX(cf_document_id) 単独の用途もカバー）
+GIN INDEX(subject_uri jsonb_path_ops)  -- 分野逆引き「この分野を設定している項目」: subject_uri @> '[{"identifier": ...}]'
 ```
 
 ### cf_association

migrations/versions/d1a2b3c4e5f6_gin_index_cf_items_subject_uri.py

@@ -0,0 +1,35 @@
+"""add GIN index on cf_items.subject_uri (jsonb_path_ops)
+
+Enables fast reverse lookup "which items set this subject" via the JSONB
+containment query ``subject_uri @> '[{"identifier": "<subject-id>"}]'``.
+``jsonb_path_ops`` is the smaller / faster GIN operator class for ``@>``.
+
+Revision ID: d1a2b3c4e5f6
+Revises: c704faa62b4f
+Create Date: 2026-06-20 00:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "d1a2b3c4e5f6"
+down_revision: Union[str, None] = "c704faa62b4f"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_index(
+        "ix_cf_items_subject_uri_gin",
+        "cf_items",
+        ["subject_uri"],
+        unique=False,
+        postgresql_using="gin",
+        postgresql_ops={"subject_uri": "jsonb_path_ops"},
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_cf_items_subject_uri_gin", table_name="cf_items")

src/models/cf_item.py

@@ -14,6 +14,13 @@ class CFItem(Base):
         UniqueConstraint("tenant_id", "identifier", name="uq_cf_items_tenant_identifier"),
         Index("ix_cf_items_tenant_document_coding", "tenant_id", "cf_document_id", "human_coding_scheme"),
         Index("ix_cf_items_document_depth", "cf_document_id", "depth"),
+        # Reverse lookup "items setting this subject": subject_uri @> '[{"identifier": ...}]'.
+        Index(
+            "ix_cf_items_subject_uri_gin",
+            "subject_uri",
+            postgresql_using="gin",
+            postgresql_ops={"subject_uri": "jsonb_path_ops"},
+        ),
     )
 
     id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)