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/schemas/oauth.py

@@ -4,6 +4,11 @@ from pydantic import BaseModel
 class OAuthCallbackPayload(BaseModel):
     code: str
     state: str | None = None
+    # When the OAuth flow originated from /accept-invite, the frontend round-trips
+    # the invite code + invited email so the backend can link the new user to the
+    # invited account instead of creating a personal one.
+    account_invite_code: str | None = None
+    invited_email: str | None = None
 
 
 class OAuthCallbackResponse(BaseModel):
@@ -11,3 +16,17 @@ class OAuthCallbackResponse(BaseModel):
     refresh_token: str
     token_type: str = "bearer"
     is_new_user: bool
+
+
+class InviteLookupResponse(BaseModel):
+    """Public response surface for GET /accounts/invites/{code}/lookup.
+
+    Returns the minimum context needed for the AcceptInvitePage:
+    account name (so we can title the card), inviter name (for the resend
+    fallback message), invited email (locked into the form), and role.
+    """
+
+    account_name: str
+    inviter_name: str
+    invited_email: str
+    role: str