chore(lint): apply ruff --fix (4443 auto-fixes)

Auto-fixed categories:
- F401: unused imports removed
- I001: import blocks sorted
- W293: trailing whitespace stripped
- UP035: deprecated typing imports replaced
- SIM: simplify suggestions applied
- ARG: unused args prefixed with underscore
- T201: print statements removed
- F841: unused variables removed
- RUF059: unpacked variables prefixed

Remaining ~1500 unfixable errors (C901, B904, N806, E402) require manual work.

Backend smoke tests: 13/13 passed.

backend/src/plugins/translate/plugin.py

@@ -5,7 +5,8 @@
 # @RATIONALE: Separate plugin avoids bloating LLMAnalysisPlugin beyond fractal limit (<400 lines).
 # @REJECTED: Extending LLMAnalysisPlugin would conflate two distinct feature domains.
 
-from typing import Dict, Any, Optional
+from typing import Any
+
 from ...core.plugin_base import PluginBase
 
 
@@ -29,7 +30,7 @@ class TranslatePlugin(PluginBase):
     def version(self) -> str:
         return "1.0.0"
 
-    def get_schema(self) -> Dict[str, Any]:
+    def get_schema(self) -> dict[str, Any]:
         return {
             "type": "object",
             "properties": {
@@ -52,7 +53,7 @@ class TranslatePlugin(PluginBase):
             "required": ["job_id"],
         }
 
-    async def execute(self, params: Dict[str, Any], context: Optional[Any] = None):
+    async def execute(self, params: dict[str, Any], context: Any | None = None):
         """Execute a translation job — implementation deferred to later phases."""
         raise NotImplementedError("TranslatePlugin.execute not yet implemented")
 # #endregion TranslatePlugin