apps/backend/app/
├── main.py          # FastAPI app, CORS, lifespan, routers
├── config.py        # Pydantic BaseSettings from env
├── database.py      # TinyDB wrapper (JSON file storage)
├── llm.py           # LiteLLM wrapper (multi-provider)
├── routers/         # API endpoint handlers
├── services/        # Business logic layer
├── schemas/         # Pydantic request/response models
└── prompts/         # LLM prompt templates (Jinja2)

from fastapi import APIRouter, HTTPException
from app.schemas.my_schema import MyRequest, MyResponse
from app.services.my_service import process_data
import logging

logger = logging.getLogger(__name__)
router = APIRouter(prefix="/api/v1", tags=["my-feature"])

@router.post("/my-endpoint", response_model=MyResponse)
async def create_thing(request: MyRequest) -> MyResponse:
    try:
        result = await process_data(request)
        return result
    except Exception as e:
        logger.error(f"Failed to create thing: {e}")
        raise HTTPException(status_code=500, detail="Operation failed. Please try again.")

from pydantic import BaseModel, Field

class MyRequest(BaseModel):
    name: str = Field(..., min_length=1, max_length=200)
    description: str | None = None

class MyResponse(BaseModel):
    id: str
    name: str
    status: str = "created"

import copy
from app.database import get_db

DEFAULT_DATA = {"sections": [], "metadata": {}}

async def get_or_create(doc_id: str) -> dict:
    db = get_db()
    existing = db.get(doc_id)
    if existing:
        return existing
    data = copy.deepcopy(DEFAULT_DATA)  # ALWAYS deepcopy mutable defaults
    db.insert(data)
    return data

from app.llm import get_completion
from app.config import settings

async def improve_text(text: str) -> str:
    prompt = f"Improve this resume text:\n\n{text}"
    result = await get_completion(
        prompt=prompt,
        model=settings.LLM_MODEL,
        api_key=settings.LLM_API_KEY,  # Pass directly, not via env
        json_mode=True,
    )
    return result

except Exception as e:
    logger.error(f"Operation failed: {e}")  # Detailed for server logs
    raise HTTPException(
        status_code=500,
        detail="Operation failed. Please try again."  # Generic for client
    )