Add NodeAction: a small, append-only log of who did what to a nodeHEADmaster

node_id required; page_id/locale/metadata nullable, for actions that
don't involve a specific revision or translation. user_id nullifies
on account deletion -- the first real DB-level foreign key to users
this app has had; every prior user_id/editor_id column is a bare,
unconstrained integer, enforced only by convention. username is
snapshotted at write time specifically so the actor's name survives
that deletion; actor_name prefers the live user (so a rename shows
up correctly for anyone still active) and only falls back to the
snapshot once the account is actually gone.

occurred_at is deliberately one field for both live-witnessed and
backfilled entries; inferred_from distinguishes the two -- nil means
witnessed at the moment it happened, populated means backfilled and
names how it was estimated, since a seeded guess and a real
observation shouldn't be presented with the same confidence.

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