Add GET /auth/api/check endpoint for unauthenticated user permission checks

Checks permissions for a user given by ?user=<UUID> query arg without
requiring a session cookie. No cookie is read or written, no DB writes.

- perm= query arg supported (same wildcard semantics as validate/forward)
- Returns valid bool + minimal ctx (user/org/role/permissions)
- Permissions are host-scoped via domain filtering, same as session_ctx
- 404 if UUID not found; valid=false if perm check fails (no 403)
- Add ApiCheckUserResponse struct to apistructs
- Add has_all_scopes() helper to permutil for scope-set-based checks

paskia/fastapi/api.py

@@ -1,6 +1,7 @@
 import logging
 from contextlib import suppress
 from datetime import UTC, datetime, timedelta
+from uuid import UUID
 
 from fastapi import (
     Depends,
@@ -20,8 +21,17 @@ from paskia.fastapi import authz, session, user
 from paskia.fastapi.response import MsgspecResponse
 from paskia.fastapi.session import AUTH_COOKIE, AUTH_COOKIE_NAME, get_client_ip
 from paskia.globals import passkey as global_passkey
-from paskia.util import hostutil, htmlutil, passphrase, userinfo
+from paskia.util import hostutil, htmlutil, passphrase, permutil, userinfo
-from paskia.util.apistructs import ApiSettings, ApiTokenInfo, ApiValidateResponse
+from paskia.util.apistructs import (
+    ApiCheckUserResponse,
+    ApiOrgContext,
+    ApiRoleContext,
+    ApiSessionContext,
+    ApiSettings,
+    ApiTokenInfo,
+    ApiUserContext,
+    ApiValidateResponse,
+)
 
 bearer_auth = HTTPBearer(auto_error=False)
 
@@ -109,6 +119,60 @@ async def validate_token(
     )
 
 
+@app.get("/check")
+async def check_user(
+    request: Request,
+    user_uuid: UUID = Query(..., alias="user"),
+    perm: list[str] = Query([]),
+):
+    """Check permissions for a user by UUID without requiring a session.
+
+    Query Params:
+    - user: UUID of the user to check.
+    - perm: repeated permission scope the user must possess (ALL required).
+
+    Returns 200 with valid=True/False and the user's effective permissions,
+    scoped to the requesting host (domain-restricted permissions are filtered).
+    Returns 404 if the user UUID does not exist.
+
+    No session cookie is read or written. Caller authentication is not required.
+    """
+    data = db.data()
+    try:
+        u = data.users[user_uuid]
+        role = u.role
+        org = role.org
+    except KeyError:
+        raise HTTPException(status_code=404, detail="User not found")
+
+    host = hostutil.normalize_host(request.headers.get("host"))
+    org_perm_uuids = {p.uuid for p in org.permissions}
+
+    effective_perms = []
+    for perm_uuid in role.permission_set:
+        if perm_uuid not in org_perm_uuids:
+            continue
+        try:
+            p = data.permissions[perm_uuid]
+        except KeyError:
+            continue
+        if p.domain is not None and p.domain != host:
+            continue
+        effective_perms.append(p)
+
+    required = " ".join(perm).split()
+    effective_scopes = {p.scope for p in effective_perms}
+    valid = permutil.has_all_scopes(effective_scopes, required)
+
+    ctx = ApiSessionContext(
+        user=ApiUserContext(uuid=u.uuid, display_name=u.display_name, theme=u.theme),
+        org=ApiOrgContext(uuid=org.uuid, display_name=org.display_name),
+        role=ApiRoleContext(uuid=role.uuid, display_name=role.display_name),
+        permissions=sorted(effective_scopes),
+    )
+    return MsgspecResponse(ApiCheckUserResponse(valid=valid, ctx=ctx))
+
+
 @app.get("/forward")
 async def forward_authentication(
     request: Request,

paskia/util/apistructs.py

@@ -233,6 +233,13 @@ class ApiValidateResponse(msgspec.Struct):
     ctx: ApiSessionContext
 
 
+class ApiCheckUserResponse(msgspec.Struct):
+    """Response struct for check-user endpoint."""
+
+    valid: bool
+    ctx: ApiSessionContext
+
+
 class ApiAdminInfo(msgspec.Struct, kw_only=True):
     """Combined admin info response."""
 

paskia/util/permutil.py

@@ -6,7 +6,7 @@ from fnmatch import fnmatchcase
 from paskia.authsession import session_ctx
 from paskia.util.hostutil import normalize_host
 
-__all__ = ["has_any", "has_all", "session_context"]
+__all__ = ["has_any", "has_all", "has_all_scopes", "session_context"]
 
 
 def _match(perms: set[str], patterns: Sequence[str]):
@@ -36,6 +36,11 @@ def has_all(ctx, patterns: Sequence[str]) -> bool:
     return all(_match(_get_effective_scopes(ctx), patterns)) if ctx else False
 
 
+def has_all_scopes(scopes: set[str], patterns: Sequence[str]) -> bool:
+    """Check that a pre-computed scope set satisfies all required patterns."""
+    return all(_match(scopes, patterns)) if patterns else True
+
+
 async def session_context(auth: str | None, host: str | None = None):
     if not auth:
         return None