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/avatar.py

@@ -9,6 +9,12 @@ def avatar_url(style: str, seed: str, size: int = 128) -> str:
     return f"/avatar/{style}/{seed}?size={size}"
 
 
+def avatar_seed(user) -> str:
+    if not user:
+        return ""
+    return user.get("avatar_seed") or user.get("username") or ""
+
+
 def generate_avatar_svg(seed: str) -> str:
     try:
         from multiavatar.multiavatar import multiavatar