♻️(backend) encapsulate token generation in a service

Encapsulate token generation logic for authenticating to the
external API in a well-scoped service.

This service can later be reused in other parts of the codebase,
especially for providing tokens required by calendar integrations.

Commit was cherry picked from #897

src/backend/core/services/jwt_token.py

@@ -0,0 +1,106 @@
+"""JWT token service."""
+
+# pylint: disable=R0913,R0917
+# ruff: noqa: PLR0913
+
+from datetime import datetime, timedelta, timezone
+from typing import Optional
+
+from django.core.exceptions import ImproperlyConfigured
+
+import jwt
+
+
+class JwtTokenService:
+    """Generic JWT token service with configurable settings."""
+
+    def __init__(
+        self,
+        secret_key: str,
+        algorithm: str,
+        issuer: str,
+        audience: str,
+        expiration_seconds: int,
+        token_type: str,
+    ):
+        """
+        Initialize the token service with custom settings.
+
+        Args:
+            secret_key: Secret key for JWT encoding/decoding
+            algorithm: JWT algorithm
+            issuer: Token issuer identifier
+            audience: Token audience identifier
+            expiration_seconds: Token expiration time in seconds
+            token_type: Token type
+
+        Raises:
+            ImproperlyConfigured: If secret_key is None or empty
+        """
+        if not secret_key:
+            raise ImproperlyConfigured("Secret key is required.")
+        if not algorithm:
+            raise ImproperlyConfigured("Algorithm is required.")
+        if not token_type:
+            raise ImproperlyConfigured("Token's type is required.")
+        if expiration_seconds is None:
+            raise ImproperlyConfigured("Expiration's seconds is required.")
+
+        self._key = secret_key
+        self._algorithm = algorithm
+        self._issuer = issuer
+        self._audience = audience
+        self._expiration_seconds = expiration_seconds
+        self._token_type = token_type
+
+    def generate_jwt(
+        self, user, scope: str, extra_payload: Optional[dict] = None
+    ) -> dict:
+        """
+        Generate an access token for the given user.
+
+        Note: any extra_payload variables named iat, exp, or user_id will
+        be overwritten by this service
+
+        Args:
+            user: User instance for whom to generate the token
+            scope: Space-separated scope string
+
+        Returns:
+            Dictionary containing access_token, token_type, expires_in, and scope optionally
+        """
+        now = datetime.now(timezone.utc)
+
+        payload = extra_payload.copy() if extra_payload else {}
+
+        payload.update(
+            {
+                "iat": now,
+                "exp": now + timedelta(seconds=self._expiration_seconds),
+                "user_id": str(user.id),
+            }
+        )
+
+        if self._issuer:
+            payload["iss"] = self._issuer
+        if self._audience:
+            payload["aud"] = self._audience
+        if scope:
+            payload["scope"] = scope
+
+        token = jwt.encode(
+            payload,
+            self._key,
+            algorithm=self._algorithm,
+        )
+
+        response = {
+            "access_token": token,
+            "token_type": self._token_type,
+            "expires_in": self._expiration_seconds,
+        }
+
+        if scope:
+            response["scope"] = scope
+
+        return response