chore: reorganize test files into domain-specific subdirectories under tests/

Split the monolithic test directory into three tiers (unit, api, e2e) with a path-mirroring directory structure. Added corresponding Makefile targets (test-unit, test-api, test-e2e) and updated all documentation references (CLAUDE.md, README.md, testing-cicd.html, testing-framework.html, testing-make.html) to reflect the new layout and naming conventions.

tests/api/auth/signup.py

@@ -0,0 +1,34 @@
+# retoor <retoor@molodetz.nl>
+
+import time
+import requests
+from tests.conftest import BASE_URL
+def _session_validation():
+    s = requests.Session()
+    name = f"val_{int(time.time() * 1000)}"
+    s.post(
+        f"{BASE_URL}/auth/signup",
+        data={
+            "username": name,
+            "email": f"{name}@t.dev",
+            "password": "secret123",
+            "confirm_password": "secret123",
+        },
+        allow_redirects=True,
+    )
+    return s
+
+
+def test_signup_short_username_rerenders_with_message(app_server):
+    r = requests.post(
+        f"{BASE_URL}/auth/signup",
+        data={
+            "username": "ab",
+            "email": "x@y.zz",
+            "password": "secret123",
+            "confirm_password": "secret123",
+        },
+        allow_redirects=False,
+    )
+    assert r.status_code == 400
+    assert "Username must be between 3 and 32 characters" in r.text