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`.

tests/unit/models.py

@@ -0,0 +1,20 @@
+# retoor <retoor@molodetz.nl>
+
+import pytest
+from pydantic import ValidationError
+
+from devplacepy.models import GameLegacyForm
+
+
+def test_game_legacy_form_accepts_valid_key():
+    assert GameLegacyForm(key="multiplier").key == "multiplier"
+
+
+def test_game_legacy_form_rejects_empty_key():
+    with pytest.raises(ValidationError):
+        GameLegacyForm(key="")
+
+
+def test_game_legacy_form_rejects_overlong_key():
+    with pytest.raises(ValidationError):
+        GameLegacyForm(key="x" * 41)