GHSA-4565-r4x7-hg8j

## Summary

A repository admin collaborator can escalate their privileges to owner-level access by exploiting an off-by-one error in the `ChangeCollaborationAccessMode` function.

## Vulnerable Code

In `internal/database/repo_collaboration.go`, line 129:

```go func (r *Repository) ChangeCollaborationAccessMode(userID int64, mode AccessMode) error { // Discard invalid input if mode <= AccessModeNone || mode > AccessModeOwner { return nil } ```

`AccessModeOwner` has value 4. The check `mode > AccessModeOwner` evaluates to `4 > 4 = false`, allowing `AccessModeOwner` to pass through. The correct check should be `mode >= AccessModeOwner`.

The web route at `internal/route/repo/setting.go:413-416` takes the mode as a raw integer from query parameters:

```go func ChangeCollaborationAccessMode(c *context.Context) { if err := c.Repo.Repository.ChangeCollaborationAccessMode( c.QueryInt64("uid"), database.AccessMode(c.QueryInt("mode"))); err != nil { ```

This allows an admin collaborator to POST `mode=4` and escalate to owner.

## Impact

A repository admin collaborator (AccessModeAdmin = 3) can escalate to owner-level access (AccessModeOwner = 4), gaining the ability to: - **Delete the repository** - **Transfer repository ownership** to another user - **Erase wiki data** - Perform all other owner-only operations

The `access` table is also updated (line 181), so the escalated permissions persist across sessions.

## Contrast

The API route at `internal/route/api/v1/repo_collaborators.go:46` uses `ParseAccessMode()` which only returns Read, Write, or Admin - never Owner. The API endpoint is not affected.

## Steps to Reproduce

1. User A creates a private repository 2. User A adds User B as a collaborator with **Admin** access (mode=3) 3. User B logs in and navigates to the repository settings collaboration page 4. User B sends a POST request: ``` POST /{owner}/{repo}/settings/collaboration/access_mode?uid={B_uid}&mode=4 ``` 5. User B now has **Owner** access - the "Danger Zone" section appears with "Delete This Repository" and "Transfer Ownership" buttons

## Suggested Fix

Change the validation in `internal/database/repo_collaboration.go` line 129 from: ```go if mode <= AccessModeNone || mode > AccessModeOwner { ``` to: ```go if mode <= AccessModeNone || mode >= AccessModeOwner { ```