Add the ability to resend unaccepted invites (#60)

Previously the only way to resend was to re-invite the same email from
scratch, creating a whole new invite row. Adds a "Resend" action next
to Revoke on each pending invite -- rotates the token and refreshes the
7-day expiry on the same row (the old link stops working the moment
it's used, same instinct as a password-reset resend), then re-sends
the invite email.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/routers/admin.py

@@ -38,6 +38,7 @@ from app.services.site_invite_service import (
     SiteInviteNotPendingError,
     create_site_invite,
     list_site_invites,
+    resend_site_invite,
     revoke_site_invite,
 )
 from app.services.smtp_settings_service import get_smtp_settings, upsert_smtp_settings
@@ -322,6 +323,22 @@ async def revoke_site_invite_endpoint(
         raise HTTPException(status_code=400, detail="Invite is no longer pending")
 
 
+@router.post("/invites/{invite_id}/resend", response_model=SiteInviteRead)
+async def resend_site_invite_endpoint(
+    invite_id: uuid.UUID,
+    request: Request,
+    current_user: User = Depends(get_current_user),
+    db: AsyncSession = Depends(get_db),
+):
+    require_site_admin(current_user)
+    try:
+        return await resend_site_invite(db, current_user, str(request.base_url), invite_id)
+    except SiteInviteNotFoundError:
+        raise HTTPException(status_code=404, detail="Invite not found")
+    except SiteInviteNotPendingError:
+        raise HTTPException(status_code=400, detail="Invite is no longer pending")
+
+
 @router.get("/settings/smtp", response_model=SmtpSettingsRead | None)
 async def get_smtp_settings_endpoint(
     current_user: User = Depends(get_current_user),