Dumps store datetime columns as RFC3339 strings (e.g. 2026-03-27T11:27:01Z).
On restore these were passed verbatim to the database, which MySQL and
MariaDB reject with 'Error 1292 Incorrect datetime value'. Parse them into
time.Time so the driver formats them for the target database.
Also guard against a nil pointer dereference when a dump contains a column
that no longer exists in the current schema - such columns are now dropped
with a warning instead of crashing.
Fixes https://github.com/go-vikunja/vikunja/issues/3375
MySQL/MariaDB/Postgres cannot bind a table name as a ? placeholder, so the non-SQLite branch failed with a syntax error. Interpolate the already-validated identifier with x.Quote (per-dialect quoting) instead. validateTableName restricts to registered table names, so this is injection-safe — the same trust model the SQLite branch already relies on. Latent bug surfaced by the new cross-engine testing webtest, which is the first to exercise this path on MySQL/MariaDB.