db: treat Go module pseudo-versions as dev builds

Untagged main-sha builds inherit a Go module pseudo-version from
runtime/debug.BuildInfo (vX.Y.Z-<timestamp>-<hash>). isDev only
filtered "", "dev", and "(devel)", so the pseudo-version was stored
in database_versions and the next real release tripped the
multi-minor upgrade guard:

    headscale version v0.29.0-beta.1 cannot be used with a database
    last used by v0.0.0-20260520093041-e4e742c776ee, upgrading more
    than one minor version at a time is not supported

Add pseudoVersionTime, a regex + time.Parse predicate covering all
three Go pseudo-version forms (v0.0.0 base, pre-release ancestor,
release ancestor), and delegate from isDev. The dev gate at
db.go:790 already prevents pseudo-versions from being written, so
already-poisoned databases self-heal on the next real-release start.

Fixes #3281