fix: allow dots in usernames; BCrypt fallback in password verifier for v1 migrations

crates/domain/src/value_objects.rs

@@ -44,9 +44,12 @@ impl Username {
         if s.is_empty() || s.len() > 32 {
             return Err(DomainError::InvalidInput("username: 1-32 chars".into()));
         }
-        if !s.chars().all(|c| c.is_alphanumeric() || c == '_') {
+        if !s
+            .chars()
+            .all(|c| c.is_alphanumeric() || c == '_' || c == '.')
+        {
             return Err(DomainError::InvalidInput(
-                "username: alphanumeric or underscore only".into(),
+                "username: alphanumeric, underscore, or dot only".into(),
             ));
         }
         Ok(Self(s))