Support transferring add-on ownership between AnkiWeb accounts

Users occasionally ask us to transfer ownership of an add-on from one AnkiWeb account to another (e.g. handing a project off to a community-maintained account). Today this has no clear path, and requests can sit unanswered.

**Goal:** make these requests fast and low-effort to resolve.

**Security requirement:** any transfer flow must require the original owner to confirm the transfer (e.g. via a confirmation link sent to their AnkiWeb email, or a similar out-of-band check). This applies to both self-serve and admin-initiated transfers.

**Proposed, in order of preference:**

1. **Self-serve transfer.** The current owner initiates a transfer from the add-on's page; the recipient confirms via a link sent to their AnkiWeb email, and the original owner confirms from theirs. Nice to have.
2. **Admin panel action.** A private admin UI for AnkiWeb maintainers to reassign an add-on's owner given the source account, destination account, and add-on ID. Must still require confirmation from the original owner's email before the reassignment takes effect. This is the immediate win — unblocks us without needing the full self-serve flow.
3. **Documented manual runbook.** As a stopgap, a short internal doc describing the DB/query steps and verification checks (including email confirmation from the original owner) so any maintainer can handle a request without paging the original author.

**Acceptance:** an incoming transfer request can be fulfilled in minutes, not weeks, without weakening ownership guarantees.

No files or tests are named. Start by tracing the existing add-on ownership, AnkiWeb account, admin-panel, database, and email-confirmation entry points, then compare the admin action with the proposed manual runbook. Done means a transfer can be fulfilled in minutes while requiring confirmation from the original owner's email and preserving ownership guarantees.

Written by the indexing model from the issue text.