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/api.py

@@ -4,6 +4,7 @@
 # To revert: cp api_monolith_backup.py api.py
 
 from fastapi import FastAPI, Request
+from fastapi.responses import JSONResponse
 from fastapi.staticfiles import StaticFiles
 from utils.logger import get_logger
 from utils.log_config import load_config as load_log_config
@@ -23,7 +24,7 @@ app = FastAPI()
 async def global_exception_handler(request: Request, exc: Exception):
     """Catch all unhandled exceptions and log them properly."""
     logger.error(f"Unhandled exception on {request.method} {request.url.path}: {exc}", exc_info=True)
-    return {"success": False, "error": "Internal server error"}
+    return JSONResponse(status_code=500, content={"success": False, "error": "Internal server error"})
 
 # ========== Logging Middleware ==========
 @app.middleware("http")