(backend) wip introduce a token exchange endpoint

src/backend/core/services/transit_code.py

@@ -0,0 +1,74 @@
+"""Service handling the lifecycle of transit codes.
+
+A transit code is an opaque, cryptographically random, single-use code
+handed to an embedded frontend (through a URL fragment) so it can obtain a
+user access token on the core API without a session cookie. The code
+carries no information by itself: everything it references (user, client)
+is stored server-side in the cache, and consumed atomically on exchange.
+"""
+
+import hashlib
+import secrets
+
+from django.conf import settings
+from django.core.cache import cache
+
+
+class TransitCodeService:
+    """Create and consume single-use transit codes."""
+
+    @staticmethod
+    def _cache_key(code):
+        """Build the cache key for a code.
+
+        The code is hashed so that a dump of the cache never reveals
+        directly usable codes.
+        """
+        digest = hashlib.sha256(code.encode("utf-8")).hexdigest()
+        return f"{settings.TRANSIT_CODE_CACHE_PREFIX}:{digest}"
+
+    def create_code(self, user, client_id="unknown"):
+        """Generate a transit code for a user, and store it.
+
+        The code expires after TRANSIT_CODE_TTL seconds.
+
+        Returns:
+            str: The opaque code to hand to the client.
+        """
+        # Default 48 random bytes -> 64 url-safe characters, 384 bits of
+        # entropy: unguessable and safe to transit through a URL fragment.
+        code = secrets.token_urlsafe(settings.TRANSIT_CODE_NBYTES)
+
+        cache.set(
+            self._cache_key(code),
+            {
+                "user_id": str(user.id),
+                "client_id": client_id,
+            },
+            timeout=settings.TRANSIT_CODE_TTL,
+        )
+
+        return code
+
+    def consume_code(self, code):
+        """Consume a transit code, enforcing single use.
+
+        The code is deleted from the cache upon consumption. `cache.delete`
+        returns whether a key was actually deleted, so if two requests race
+        on the same code, only one of them wins.
+
+        Returns:
+            dict | None: The data stored at creation time ('user_id',
+                'client_id'), or None if the code is unknown, expired or
+                already consumed.
+        """
+        if not code:
+            return None
+
+        key = self._cache_key(code)
+        data = cache.get(key)
+
+        if data is None or not cache.delete(key):
+            return None
+
+        return data