Replace the hardcoded validTableNames allowlist with a regexp that
only allows [a-z0-9_] characters. This prevents SQL injection without
requiring maintenance of a separate allowlist when new tables are added.
Addresses review feedback from @sneak on PR #32.
CompressStream had both a defer w.Close() and an explicit w.Close() call,
causing the compressor and encryptor to be closed twice. The second close
on the zstd encoder returns an error, and the age encryptor may write
duplicate finalization bytes, potentially corrupting the output stream.
Use a closed flag to prevent the deferred close from running after the
explicit close succeeds.
The getTableCount method used fmt.Sprintf to interpolate a table name directly
into a SQL query. While currently only called with hardcoded names, this is a
dangerous pattern. Added an allowlist of valid table names and return an error
for unrecognized names.