Migrations : fix ADD CONSTRAINT IF NOT EXISTS (syntaxe PostgreSQL invalide)

Remplace par un bloc DO $$ IF NOT EXISTS ... $$ pour le FK circulaire
item_versions→document_items. Testé localement sur pg-test : 5 migrations
OK + idempotence OK.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/alembic/versions/2026_04_23_0000-0b9c1d2e3f4a_initial_schema.py

@@ -232,9 +232,18 @@ def upgrade() -> None:
     """)
 
     op.execute("""
+    DO $$
+    BEGIN
+        IF NOT EXISTS (
+            SELECT 1 FROM information_schema.table_constraints
+            WHERE constraint_name = 'item_versions_item_id_fkey'
+            AND table_name = 'item_versions'
+        ) THEN
             ALTER TABLE item_versions
-        ADD CONSTRAINT IF NOT EXISTS item_versions_item_id_fkey
-        FOREIGN KEY (item_id) REFERENCES document_items(id)
+                ADD CONSTRAINT item_versions_item_id_fkey
+                FOREIGN KEY (item_id) REFERENCES document_items(id);
+        END IF;
+    END $$
     """)
 
     op.execute("""