FIX: Remove envelope response pattern and add Pydantic response models

[YektaY]

Description

Note: This PR builds off of #75 and should not be reviewed until that has be reviewed and merged

• Drop the {errorCode, errorMessage, payload} envelope from every REST response. Endpoints now return their DTO (or primitive) directly.
• Add response_model= to every handler in app/api/v1/ so Swagger and /openapi.json render accurate, per-endpoint response schemas instead of {}.
• Add missing response DTOs for endpoints that previously returned ad-hoc dicts
• Replace raise APIException(code, msg, status) with raise HTTPException(status_code=code, detail=msg) across handlers and app/dependencies.py. Error bodies now follow FastAPI's default {"detail": "..."} shape.
• Delete app/api/responses.py (the success_response / error_response helpers and APIException class), ApiResultResponse from app/schemas/common.py, and the matching exception handler in app/main.py.
• Update tests

Motivation

The envelope pattern was tracked as a bug in #32 and react-squirrel#77. Two problems with the old design:

1. OpenAPI/Swagger docs were useless for responses. Because every handler was typed -> dict, FastAPI couldn't generate accurate response examples — /docs showed generic {} for every endpoint. With response_model= attached, Swagger now shows the actual field shape for every response, and any consumer regenerating types from /openapi.json gets correct TypeScript interfaces.
2. Redundant wrapping/unwrapping. HTTP already has status codes for success/failure. The envelope forced both sides to hand-serialize (success_response(x) on the backend) and hand-deserialize (response.data.payload on the frontend) for no added signal. Removing it simplifies both ends and eliminates a class of bugs where handlers forgot to wrap.

The frontend work to drop the apiClient.ts unwrap layer is react-squirrel#77 and needs to deploy with with this change.

Closes #32

Response shape diff (example)

Before — GET /v1/snapshots:

{
  "errorCode": 0,
  "errorMessage": null,
  "payload": [{ "id": "...", "title": "...", "pvCount": 42 }]
}

After — GET /v1/snapshots:

[{ "id": "...", "title": "...", "pvCount": 42 }]

Pre-merge checklist

• Code works interactively
• Code contains descriptive docstrings
• New/changed functions and methods are covered in the test suite where possible
• Test suite passes locally
• Test suite passes on GitHub Actions