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>

datasette/renderer.py

@@ -20,7 +20,7 @@ def convert_specific_columns_to_json(rows, columns, json_cols):
             if column in json_cols:
                 try:
                     value = json.loads(value)
-                except (TypeError, ValueError) as e:
+                except (TypeError, ValueError):
                     pass
             new_row.append(value)
         new_rows.append(new_row)