Add ty type checker with all rules as errors (#239)

#### Problem
The project lacked ty type checker configuration, leaving potential type
issues undetected.

#### Solution
Add `[tool.ty.rules] all = "error"` to pyproject.toml and add a missing
`@override` decorator in `_main.py` to satisfy ty's checks.

Author: mxr

pyproject.toml

@@ -131,3 +131,6 @@ commands = [
   ["coverage", "report"],
 ]
 dependency_groups = ["dev"]
+
+[tool.ty.rules]
+all = "error"

sqlite_export_for_ynab/_main.py

@@ -14,6 +14,7 @@
 from typing import Any
 from typing import Literal
 from typing import overload
+from typing import override
 from typing import TYPE_CHECKING
 
 import aiosqlite
@@ -67,6 +68,7 @@ def __init__(self, separator: str = "/", table_column: Column | None = None):
             self.separator = separator
             super().__init__(table_column=table_column)
 
+        @override
         def render(self, task: Task) -> Text:
             """Show completed/total."""
             completed = int(task.completed)