feat: add resend capability for platform and account invite codes

Revoke-and-recreate flow for both invite systems with email delivery
via Resend API. Includes account invite email template and audit logging.

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

docs/plans/2026-02-12-resend-invite-codes.md

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+# Resend Invite Codes — Design
+
+**Date**: 2026-02-12
+
+## Summary
+
+Add a "Resend" capability to both invite systems (platform invite codes and account invites). Resending revokes the old code and generates a fresh one, then emails it to the recipient.
+
+## Backend
+
+### Platform Invite Codes
+
+**New endpoint**: `POST /api/v1/invites/{code}/resend` (admin-only)
+
+1. Look up existing invite by code
+2. Reject if already used (409 Conflict)
+3. Delete the old invite
+4. Create a new invite with the same properties (email, plan, trial_days, note, expiration recalculated from now)
+5. Send email via `EmailService.send_invite_email()`
+6. Log audit event
+7. Return the new invite
+
+### Account Invites
+
+**New endpoint**: `POST /api/v1/accounts/me/invites/{invite_id}/resend` (owner-only)
+
+1. Look up existing invite by ID
+2. Reject if already used (409 Conflict)
+3. Delete the old invite
+4. Create a new invite with the same email, role, and fresh expiration
+5. Send email via new `EmailService.send_account_invite_email()`
+6. Log audit event
+7. Return the new invite
+
+### New Email Method: `send_account_invite_email()`
+
+Added to `EmailService` in `backend/app/core/email.py`.
+
+- **Parameters**: `to_email`, `code`, `account_name`, `role`, `signup_url`
+- **Subject**: "You've been invited to join [Account Name] on ResolutionFlow"
+- **Body**: Same dark monochrome HTML template as platform invites, with:
+  - "You've been invited to join **[Account Name]** as an **Engineer/Viewer**"
+  - Prominent invite code display (same style)
+  - No plan/trial section
+  - "Create Your Account" CTA button
+- **Returns** `bool` — best-effort, never raises
+
+## Frontend
+
+### Platform Invite Codes (`InviteCodesPage.tsx`)
+
+- Resend button in actions column (next to copy/delete)
+- Only visible for unused, non-expired codes with an email address
+- Calls `POST /api/v1/invites/{code}/resend`
+- Shows success toast with new code, refreshes list
+- Loading state on button during API call
+
+### Account Invites (`AccountSettingsPage.tsx`)
+
+- Resend button next to each pending invite
+- Only visible for unused, non-expired invites
+- Calls `POST /api/v1/accounts/me/invites/{invite_id}/resend`
+- Shows success toast, refreshes list
+- Loading state on button during API call
+
+## Files to Modify
+
+- `backend/app/core/email.py` — add `send_account_invite_email()` + HTML template
+- `backend/app/api/endpoints/invite.py` — add resend endpoint for platform codes
+- `backend/app/api/endpoints/accounts.py` — add resend endpoint for account invites
+- `frontend/src/pages/admin/InviteCodesPage.tsx` — add resend button
+- `frontend/src/pages/AccountSettingsPage.tsx` — add resend button
+- `frontend/src/api/admin.ts` — add resend API call
+- `frontend/src/api/accounts.ts` — add resend API call