feat: self-serve signup Phase 2 (frontend cutover) (#162)

Co-authored-by: Michael Chihlas <michael@resolutionflow.com>
Co-committed-by: Michael Chihlas <michael@resolutionflow.com>

backend/app/api/endpoints/account_invite_lookup.py

@@ -0,0 +1,54 @@
+"""Public endpoint for resolving an account invite code into display info.
+
+Mounted as a public route (no tenant context, no auth) — used by the
+/accept-invite page on the frontend so an invitee can see what account they
+are about to join before they sign up. Uses the BYPASSRLS admin session
+factory because account_invites is account-scoped under Phase 4 RLS but the
+caller has no tenant identity yet.
+"""
+
+from typing import Annotated
+
+from fastapi import APIRouter, Depends, HTTPException
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import joinedload
+
+from app.core.admin_database import get_admin_db
+from app.models.account_invite import AccountInvite
+from app.schemas.oauth import InviteLookupResponse
+
+router = APIRouter(prefix="/accounts", tags=["account-invite-lookup"])
+
+
+@router.get("/invites/{code}/lookup", response_model=InviteLookupResponse)
+async def lookup_invite(
+    code: str,
+    db: Annotated[AsyncSession, Depends(get_admin_db)],
+) -> InviteLookupResponse:
+    """Return minimal display data for a valid (unused, unexpired, not revoked)
+    invite. Returns 404 with `invite_invalid_or_expired_or_revoked` for any
+    invalid state — the AcceptInvitePage shows a single "ask the inviter to
+    resend" message regardless of which condition failed (anti-enumeration)."""
+    result = await db.execute(
+        select(AccountInvite)
+        .where(AccountInvite.code == code)
+        .options(
+            joinedload(AccountInvite.account),
+            joinedload(AccountInvite.invited_by),
+        )
+    )
+    invite = result.scalar_one_or_none()
+
+    if invite is None or not invite.is_valid:
+        raise HTTPException(
+            status_code=404,
+            detail={"error": "invite_invalid_or_expired_or_revoked"},
+        )
+
+    return InviteLookupResponse(
+        account_name=invite.account.name,
+        inviter_name=invite.invited_by.name,
+        invited_email=invite.email,
+        role=invite.role,
+    )