feat: add per-user avatar seed regeneration with irreversible random avatar replacement

Implement a new `avatar_seed` column on the users table that overrides the username-based seed for Multiavatar generation. Introduce a null-safe `avatar_seed(user)` choke point in `avatar.py` that resolves `user.get("avatar_seed") or user.get("username")`, registered as a Jinja global so every render site (`_avatar_link.html`, `avatar_url(...)` calls, SEO `og_image`, issues ad-hoc dicts, devRant payload/PNG) propagates a regenerated seed. Add `POST /profile/{username}/regenerate-avatar` endpoint (owner-or-admin only) that writes a fresh `generate_uid()` to `avatar_seed`, invalidates the target's user cache, and audits `profile.avatar.regenerate`. The previous seed is overwritten and never stored, making regeneration irreversible. Document the feature in `AGENTS.md` and `README.md`, add the API endpoint to `docs_api.py`, and include the `regenerate_avatar` Devii tool in `CONFIRM_REQUIRED`.

devplacepy/templates/base.html

@@ -103,7 +103,7 @@
                 {% endif %}
                 <div class="topnav-user-dropdown">
                     <button type="button" class="topnav-user" aria-label="User menu" aria-expanded="false" aria-controls="user-menu">
-                        <img src="{{ avatar_url('multiavatar', user['username'], 32) }}" class="avatar-img avatar-sm" alt="{{ user['username'] }}" loading="lazy">
+                        <img src="{{ avatar_url('multiavatar', avatar_seed(user), 32) }}" class="avatar-img avatar-sm" alt="{{ user['username'] }}" loading="lazy">
                         <div class="topnav-user-info">
                             <span class="topnav-user-name">{{ user['username'] }}</span>
                             <span class="topnav-user-level">Level {{ user.get('level', 1) }}</span>
@@ -160,7 +160,7 @@
             {% endif %}
             <div class="topnav-mobile-divider"></div>
             <div class="topnav-mobile-user">
-                <img src="{{ avatar_url('multiavatar', user['username'], 32) }}" class="avatar-img avatar-sm" alt="{{ user['username'] }}" loading="lazy">
+                <img src="{{ avatar_url('multiavatar', avatar_seed(user), 32) }}" class="avatar-img avatar-sm" alt="{{ user['username'] }}" loading="lazy">
                 <div class="topnav-mobile-user-info">
                     <span>{{ user['username'] }}</span>
                     <span class="topnav-mobile-user-level">Level {{ user.get('level', 1) }}</span>