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/avatar.py

@@ -1,7 +1,7 @@
 # retoor <retoor@molodetz.nl>
 
 import requests
-from devplacepy.avatar import avatar_url, generate_avatar_svg
+from devplacepy.avatar import avatar_seed, avatar_url, generate_avatar_svg
 from tests.conftest import BASE_URL
 
 
@@ -76,6 +76,27 @@ def test_avatar_url_with_different_seeds():
     assert url1 != url2
 
 
+def test_avatar_seed_prefers_avatar_seed_field():
+    assert avatar_seed({"username": "alice", "avatar_seed": "seed-xyz"}) == "seed-xyz"
+
+
+def test_avatar_seed_falls_back_to_username():
+    assert avatar_seed({"username": "alice"}) == "alice"
+
+
+def test_avatar_seed_falls_back_when_seed_blank():
+    assert avatar_seed({"username": "alice", "avatar_seed": ""}) == "alice"
+    assert avatar_seed({"username": "alice", "avatar_seed": None}) == "alice"
+
+
+def test_avatar_seed_none_user():
+    assert avatar_seed(None) == ""
+
+
+def test_avatar_seed_empty_user():
+    assert avatar_seed({}) == ""
+
+
 def test_generate_avatar_svg_empty_seed_uses_fallback():
     svg = generate_avatar_svg("")
     assert isinstance(svg, str)