feat: add proper HTTP status codes to all API error responses

- 217 error returns across 18 route files + api.py now use JSONResponse
  with appropriate HTTP status codes instead of returning HTTP 200
- Status code distribution: 500 (121), 400 (39), 503 (28), 404 (24), 409 (3), 502 (2)
- Fixed language.py tuple-return bug (was serializing as JSON array)
- Fixed bare except clauses in bipolar_mode.py and voice.py
- Body-level error schemas preserved (status/error + success/error patterns)
  so web UI continues working without changes
- chat.py (SSE) unchanged: errors sent within stream protocol
- All 170 tests pass

bot/routes/language.py

@@ -1,6 +1,7 @@
 """Language mode routes."""
 
 from fastapi import APIRouter
+from fastapi.responses import JSONResponse
 import globals
 from utils.logger import get_logger
 
@@ -53,7 +54,7 @@ def toggle_language_mode():
 def set_language_mode(language: str = "english"):
     """Set language mode to either 'english' or 'japanese'"""
     if language.lower() not in ["english", "japanese"]:
-        return {"error": f"Invalid language mode '{language}'. Use 'english' or 'japanese'."}, 400
+        return JSONResponse(status_code=400, content={"status": "error", "message": f"Invalid language mode '{language}'. Use 'english' or 'japanese'."})
 
     globals.LANGUAGE_MODE = language.lower()
     model_used = globals.JAPANESE_TEXT_MODEL if language.lower() == "japanese" else globals.TEXT_MODEL