fix(migration): use DO/EXCEPTION for idempotent enum creation in b035

Replace _enum_exists() helper (which had connection context issues in
Alembic) with PostgreSQL DO $$ BEGIN ... EXCEPTION WHEN duplicate_object
THEN NULL; END $$ blocks, which are truly idempotent regardless of
transaction state.

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

backend/alembic/versions/b035_ownership_queue.py

@@ -21,33 +21,28 @@ def _table_exists(table_name: str) -> bool:
     return conn.dialect.has_table(conn, table_name)
 
 
-def _column_exists(table_name: str, column_name: str) -> bool:
-    conn = op.get_bind()
-    insp = sa.inspect(conn)
-    cols = [c["name"] for c in insp.get_columns(table_name)]
-    return column_name in cols
-
-
-def _enum_exists(enum_name: str) -> bool:
-    conn = op.get_bind()
-    result = conn.execute(
-        sa.text("SELECT 1 FROM pg_type WHERE typname = :name"), {"name": enum_name}
-    ).fetchone()
-    return result is not None
-
-
 def upgrade() -> None:
-    # ── Enums ────────────────────────────────────────────────────────────────
-    if not _enum_exists("queue_priority"):
-        op.execute("CREATE TYPE queue_priority AS ENUM ('critical', 'high', 'medium', 'low')")
-    if not _enum_exists("queue_status"):
-        op.execute("CREATE TYPE queue_status AS ENUM ('pending', 'in_progress', 'completed', 'dismissed')")
-    if not _enum_exists("queue_reason"):
-        op.execute(
-            "CREATE TYPE queue_reason AS ENUM ("
-            "'validation_expired', 'infra_change', 'osint_alert', "
-            "'mitre_update', 'rule_modified', 'low_confidence', 'manual')"
-        )
+    # ── Enums (idempotent via DO/EXCEPTION) ──────────────────────────────────
+    op.execute("""
+        DO $$ BEGIN
+            CREATE TYPE queue_priority AS ENUM ('critical', 'high', 'medium', 'low');
+        EXCEPTION WHEN duplicate_object THEN NULL;
+        END $$;
+    """)
+    op.execute("""
+        DO $$ BEGIN
+            CREATE TYPE queue_status AS ENUM ('pending', 'in_progress', 'completed', 'dismissed');
+        EXCEPTION WHEN duplicate_object THEN NULL;
+        END $$;
+    """)
+    op.execute("""
+        DO $$ BEGIN
+            CREATE TYPE queue_reason AS ENUM (
+                'validation_expired', 'infra_change', 'osint_alert',
+                'mitre_update', 'rule_modified', 'low_confidence', 'manual');
+        EXCEPTION WHEN duplicate_object THEN NULL;
+        END $$;
+    """)
 
     # ── technique_ownerships ─────────────────────────────────────────────────
     if not _table_exists("technique_ownerships"):