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_dispute_attachment_public_hash.py

@@ -28,7 +28,7 @@ async def run_migration():
             logger.info("Adding 'public_hash' column to dispute_attachments table")
             await conn.execute(text("""
                 ALTER TABLE dispute_attachments
-                ADD COLUMN public_hash VARCHAR(64) UNIQUE;
+                ADD COLUMN IF NOT EXISTS public_hash VARCHAR(64) UNIQUE;
             """))
 
             # Create index for fast lookups