fix: remove unused imports and bare exception aliases (pyflakes F401/F841)

Remove unused imports across 86 files in src/, web_interface/, test/,
and scripts/ using autoflake. No logic changes — only dead import
statements and unused names in from-imports are removed.

Also remove bare exception aliases where the variable is never
referenced in the handler body:
- src/cache/disk_cache.py: except (IOError, OSError, PermissionError) as e
- src/cache_manager.py: except (OSError, IOError, PermissionError) as perm_error
- src/plugin_system/resource_monitor.py: except Exception as e
- web_interface/app.py: except Exception as read_err

86 files changed, 205 lines removed, 18 pre-existing test failures unchanged.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/startup_validator.py

@@ -6,8 +6,7 @@ Fails fast with clear error messages to prevent runtime issues.
 """
 
 import os
-import logging
-from typing import Dict, Any, List, Optional, Tuple
+from typing import Any, List, Optional, Tuple
 from pathlib import Path
 from src.exceptions import ConfigError, PluginError, CacheError
 from src.logging_config import get_logger