feat(datastore): add last_pulled_at column to tags table

What

Adds a nullable last_pulled_at timestamptz column to the tags table. This is the foundational schema change for pull-activity-based cleanup policies — recording when a tag was last pulled.

No backfill: existing tags stay NULL until populated by a later change. Nothing reads or writes the column yet; this MR is schema-only.

Prerequisite for gitlab#600850. Related to #2311.

Scope

This MR was intentionally reduced to step 1: the column only, per review feedback (thread). The write path and API exposure follow as separate MRs so each is small and easy to review/roll back:

• This MR — add the last_pulled_at column (premigration + regenerated structure.sql).
• Follow-up: write path — populate the column off the hot manifest-pull path using an async, batched approach (in-memory aggregation + periodic bulk UPDATE), rather than a synchronous per-pull statement.
• Follow-up: API — expose last_pulled_at on the GitLab v1 tags/list and tags/detail responses once the column is populated.

Changes

• premigrations/20260529120000_add_tags_last_pulled_at_column.go — ALTER TABLE tags ADD COLUMN IF NOT EXISTS last_pulled_at timestamp WITH time zone (idempotent up/down).
• structure.sql — regenerated to include the new column.

Open question

The column is named last_pulled_at to match existing pull terminology in the handlers and queue bridge. Flagging in case last_downloaded_at is preferred — easiest to settle before it ships.

Testing

go build ./... and go vet pass. The structure.sql-consistency test runs in CI against a registry Postgres.