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/gpu.py

@@ -1,6 +1,7 @@
 """GPU selection routes."""
 
 from fastapi import APIRouter, Request
+from fastapi.responses import JSONResponse
 from utils.logger import get_logger
 
 logger = get_logger('api')
@@ -22,7 +23,7 @@ async def select_gpu(request: Request):
     gpu = data.get("gpu", "nvidia").lower()
 
     if gpu not in ["nvidia", "amd"]:
-        return {"status": "error", "message": "Invalid GPU selection. Must be 'nvidia' or 'amd'"}
+        return JSONResponse(status_code=400, content={"status": "error", "message": "Invalid GPU selection. Must be 'nvidia' or 'amd'"})
 
     try:
         from config_manager import config_manager
@@ -32,7 +33,7 @@ async def select_gpu(request: Request):
             logger.info(f"GPU Selection: Switched to {gpu.upper()} GPU")
             return {"status": "ok", "message": f"Switched to {gpu.upper()} GPU", "gpu": gpu}
         else:
-            return {"status": "error", "message": "Failed to save GPU state"}
+            return JSONResponse(status_code=500, content={"status": "error", "message": "Failed to save GPU state"})
     except Exception as e:
         logger.error(f"GPU Selection Error: {e}")
-        return {"status": "error", "message": str(e)}
+        return JSONResponse(status_code=500, content={"status": "error", "message": str(e)})