Players cannot edit GM-authored public (group) wiki notes

[hunter-read]

Summary

Split off from #230. A player can only edit a wiki page they authored; a group-visible page the GM created is read-only to players even though everyone can see it.

Backend, can_edit_page:

def can_edit_page(page, campaign, user):
    """Owner edits anything; a member may edit a page they authored."""
    if campaign.owner_id == user.id:
        return True
    return page.created_by_id == user.id

So a shared "party knowledge base" page started by the GM can't be contributed to by players — they can read it but not add to it.

Requested

Allow players to edit GM-authored group-visible pages so the wiki can be a collaborative knowledge base (as described in #230).

Notes / open questions

• Scope which pages become player-editable: probably group visibility only. members-shared and gm pages should stay owner-controlled (or editable only by shared members — to be decided).
• Interacts with GM secrets (GM secret deleted on shared notes #230): a player editing a GM-authored page will hit the same masked-secret flow. The merge_gm_secrets / ⟦GM·N⟧ marker mechanism added in GM secret deleted on shared notes #230 already covers preserving secrets on such edits — verify it holds for GM-authored pages too.
• Consider an opt-in ("allow players to edit") vs. always-on for group pages.
• Update can_edit_page, the can_edit flags returned by the list/detail endpoints, and PageEditor gating. Docs: docs/api.md and the campaigns guide.

Follow-up to #230.