Add a configurable "mark" flag to taggings and subscriptions

[maebeale]

🤖 suggested review level: 3 Read 📖 additive columns + inline Turbo actions across two index flows; reuses existing JS

Lets staff tag people and then check them off the list — marking a topic subscription or a staff tagging right on the index, with a per-row notes box. The mark means whatever each topic/tag decides ("Confirmed", "Completed", "Tracked", …).

How it works

• Each staff tag and topic carries an optional mark_label — the word for its mark (editable on Manage staff tags / Manage topics). Defaults to "Marked".
• Each staff tagging and subscription carries a marked boolean.
• Meaning lives in the per-tag/topic label; the stored column stays generic.

On the index (the main workflow)

• Inline mark slider per row toggles marked in place via Turbo (toggle_marked → turbo_stream replacing just the cell) — the same no-JS pattern as certificate-issued.
• Notes column per row: a box that autosaves on blur through the existing autosave Stimulus controller (no new JS) to a save_note action, which upserts the row's latest comment — so it shows up in that row's comment log.
• Mark column is sortable; its header shows the filtered tag/topic's own label when narrowed to one.
• All / Yes / No mark filter, inside a filter card with the shared filter-icon header.
• Staff taggings index gains a Staff tags nav link.

Setting the mark elsewhere

• A slider (existing peer-checked switch style) on the new/edit staff-tagging forms; a checkbox on the subscription edit form.

Notes for the reviewer

• Inline note semantics: the box edits the row's latest comment, or creates one if none — a quick jot lands in the same comment log the edit page shows. Flag if you'd rather it always create a fresh comment.
• Comments are eager-loaded on both indexes to avoid an N+1 from the notes column.
• One existing subscriptions spec was narrowed: it broadly asserted nothing in the results frame targets the frame, but the sort link legitimately does. It now checks the status toggle specifically still breaks out to _top.
• Migration is additive (marked default false + index; mark_label nullable) — no backfill.

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