Fix migration transaction poisoning: ADD COLUMN IF NOT EXISTS

Migrations used try/except around ADD COLUMN inside a single engine.begin()
block. When a 'column already exists' error was caught, asyncpg left the
transaction in aborted state, causing all subsequent DDL in the block to fail
with InFailedSQLTransactionError. Replace with IF NOT EXISTS to prevent the
error entirely.

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

backend/migrations/add_supplier_skip_dext.py

@@ -13,7 +13,7 @@ async def migrate():
             await conn.execute(text(
                 """
                 ALTER TABLE suppliers
-                ADD COLUMN skip_dext BOOLEAN NOT NULL DEFAULT FALSE
+                ADD COLUMN IF NOT EXISTS skip_dext BOOLEAN NOT NULL DEFAULT FALSE
                 """
             ))
             print("✓ Added skip_dext column to suppliers table")