Add artdag to OAuth clients + POST /auth/oauth/token endpoint

Standard HTTP token exchange for clients that don't share the coop DB.
Returns user_id, username, display_name, grant_token in exchange for
a valid authorization code.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bp/auth/routes.py

@@ -43,7 +43,7 @@ from .services import (
 SESSION_USER_KEY = "uid"
 ACCOUNT_SESSION_KEY = "account_sid"
 
-ALLOWED_CLIENTS = {"blog", "market", "cart", "events", "federation"}
+ALLOWED_CLIENTS = {"blog", "market", "cart", "events", "federation", "artdag"}
 
 
 def register(url_prefix="/auth"):
@@ -121,6 +121,69 @@ def register(url_prefix="/auth"):
             f"&account_did={account_did}"
         )
 
+    # --- OAuth2 token exchange (for external clients like artdag) -------------
+
+    @auth_bp.post("/oauth/token")
+    @auth_bp.post("/oauth/token/")
+    async def oauth_token():
+        """Exchange an authorization code for user info + grant token.
+
+        Used by clients that don't share the coop database (e.g. artdag).
+        Accepts JSON: {code, client_id, redirect_uri}
+        Returns JSON: {user_id, username, display_name, grant_token}
+        """
+        data = await request.get_json()
+        if not data:
+            return jsonify({"error": "invalid_request"}), 400
+
+        code = data.get("code", "")
+        client_id = data.get("client_id", "")
+        redirect_uri = data.get("redirect_uri", "")
+
+        if client_id not in ALLOWED_CLIENTS:
+            return jsonify({"error": "invalid_client"}), 400
+
+        now = datetime.now(timezone.utc)
+
+        async with get_session() as s:
+            async with s.begin():
+                result = await s.execute(
+                    select(OAuthCode)
+                    .where(OAuthCode.code == code)
+                    .with_for_update()
+                )
+                oauth_code = result.scalar_one_or_none()
+
+                if not oauth_code:
+                    return jsonify({"error": "invalid_grant"}), 400
+
+                if oauth_code.used_at is not None:
+                    return jsonify({"error": "invalid_grant"}), 400
+
+                if oauth_code.expires_at < now:
+                    return jsonify({"error": "invalid_grant"}), 400
+
+                if oauth_code.client_id != client_id:
+                    return jsonify({"error": "invalid_grant"}), 400
+
+                if oauth_code.redirect_uri != redirect_uri:
+                    return jsonify({"error": "invalid_grant"}), 400
+
+                oauth_code.used_at = now
+                user_id = oauth_code.user_id
+                grant_token = oauth_code.grant_token
+
+                user = await s.get(User, user_id)
+                if not user:
+                    return jsonify({"error": "invalid_grant"}), 400
+
+                return jsonify({
+                    "user_id": user_id,
+                    "username": user.email or "",
+                    "display_name": user.name or "",
+                    "grant_token": grant_token,
+                })
+
     # --- Grant verification (internal endpoint) ------------------------------
 
     @auth_bp.get("/internal/verify-grant")