Allow dots in usernames (e.g. user.test)

Added . to the username character whitelist regex. No security
reason to exclude it — dots are standard in usernames.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/schemas/auth.py

@@ -26,8 +26,8 @@ def _validate_username(v: str) -> str:
     v = v.strip().lower()
     if not 3 <= len(v) <= 50:
         raise ValueError("Username must be 3–50 characters")
-    if not re.fullmatch(r"[a-z0-9_\-]+", v):
-        raise ValueError("Username may only contain letters, numbers, _ and -")
+    if not re.fullmatch(r"[a-z0-9_.\-]+", v):
+        raise ValueError("Username may only contain letters, numbers, _ . and -")
     return v