refactor: improve PostgreSQL migration code quality

Why this change is needed:
1. Added clarifying comments to _pg_migrate_workspace_data() parameter handling
2. Removed dead code from PGDocStatusStorage.initialize() that was never executed

Changes:

1. PostgreSQL Migration Parameter Documentation (lightrag/kg/postgres_impl.py:2240-2241):
   - Added comments explaining dict rebuild for correct value ordering
   - Clarifies that Python 3.7+ dict insertion order is relied upon
   - Documents that execute() converts dict to tuple via .values()

2. Dead Code Removal (lightrag/kg/postgres_impl.py:3061-3062):
   - Removed unreachable table creation code from PGDocStatusStorage.initialize()
   - Table is already created by PostgreSQLDB.initdb() during initialization
   - This code path was never executed as table always exists before initialize() is called
   - Added NOTE comment explaining where table creation actually happens

Impact:
- No functional changes - only code clarification and cleanup
- Reduces maintenance burden by removing unreachable code
- Improves code readability with better documentation

Testing:
- All 14 PostgreSQL migration tests pass
- All 5 UnifiedLock safety tests pass
- Pre-commit checks pass (ruff-format, ruff)

Author: BukeLy

lightrag/kg/postgres_impl.py

@@ -2237,6 +2237,8 @@ async def _pg_migrate_workspace_data(
                 VALUES ({placeholders})
                 ON CONFLICT (workspace, id) DO NOTHING
             """
+            # Rebuild dict in columns order to ensure values() matches placeholders order
+            # Python 3.7+ dicts maintain insertion order, and execute() uses tuple(data.values())
             values = {col: row_dict[col] for col in columns}
             await db.execute(insert_query, values)
 
@@ -3058,11 +3060,8 @@ async def initialize(self):
                 # Use "default" for compatibility (lowest priority)
                 self.workspace = "default"
 
-            # Create table if not exists
-            table_name = namespace_to_table_name(self.namespace)
-            table_exists = await _pg_table_exists(self.db, table_name)
-            if not table_exists:
-                await _pg_create_table(self.db, table_name, table_name)
+            # NOTE: Table creation is handled by PostgreSQLDB.initdb() during initialization
+            # No need to create table here as it's already created in the TABLES dict
 
     async def finalize(self):
         if self.db is not None: