Fix type annotation bugs and remove unused imports

This fixes issues introduced by the ruff commit e57f391a which converted
Optional[x] to x | None:

- Fixed datasette/app.py line 1024: Dict[id | str, Dict] -> Dict[int | str, Dict]
  (was using id built-in function instead of int type)
- Fixed datasette/app.py line 1074: Optional["Resource"] -> "Resource" | None
- Added 'from __future__ import annotations' for Python 3.10 compatibility
- Added TYPE_CHECKING blocks to avoid circular imports
- Removed dead code (unused variable assignments) from cli.py and views
- Removed unused imports flagged by ruff across multiple files
- Fixed test fixtures: moved app_client fixture imports to conftest.py
  (fixed 71 test errors caused by fixtures not being registered)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

tests/test_html.py

@@ -1,4 +1,3 @@
-from asgi_csrf import Errors
 from bs4 import BeautifulSoup as Soup
 from datasette.app import Datasette
 from datasette.utils import allowed_pragmas