♻️ Simplify CORS configuration (#2429)

## AI Disclaimer

Using Codex with gpt-5.6-sol. Reviewed manually.

backend/app/core/config.py

@@ -1,9 +1,7 @@
 import warnings
-from typing import Annotated, Any, Literal, Self
+from typing import Literal, Self
 
 from pydantic import (
-    AnyUrl,
-    BeforeValidator,
     EmailStr,
     HttpUrl,
     PostgresDsn,
@@ -13,14 +11,6 @@ from pydantic import (
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
 
-def parse_cors(v: Any) -> list[str] | str:
-    if isinstance(v, str) and not v.startswith("["):
-        return [i.strip() for i in v.split(",") if i.strip()]
-    elif isinstance(v, list | str):
-        return v
-    raise ValueError(v)
-
-
 class Settings(BaseSettings):
     model_config = SettingsConfigDict(
         # Use top level .env file (one level above ./backend/)
@@ -35,17 +25,6 @@ class Settings(BaseSettings):
     FRONTEND_HOST: str = "http://localhost:5173"
     FASTAPI_ENV: Literal["development"] | None = None
 
-    BACKEND_CORS_ORIGINS: Annotated[
-        list[AnyUrl] | str, BeforeValidator(parse_cors)
-    ] = []
-
-    @computed_field  # type: ignore[prop-decorator]
-    @property
-    def all_cors_origins(self) -> list[str]:
-        return [str(origin).rstrip("/") for origin in self.BACKEND_CORS_ORIGINS] + [
-            self.FRONTEND_HOST
-        ]
-
     PROJECT_NAME: str
     SENTRY_DSN: HttpUrl | None = None
     POSTGRES_SERVER: str