db.schema_version / extraction_version swallow every sqlite3.Error and return None #128

Found by the code-reviewer agent on #123. This is the same family as #110.

db.schema_version and db.extraction_version catch every sqlite3.Error and return None. On a locked or unreadable index with a schema pin set, the user gets registry pins codegraph schema N but the index reports no schema_versions row — cannot verify column semantics. That message is misleading, because the row may well exist.

The run still blocks, so this is not a false green. The diagnosis is wrong, though, and it sends the user to look at the schema rather than the lock.

Fix: return None only for "no such table/column". Let other errors propagate, or report them as what they are.