Link stories and workshops as a many-to-many (both sides)

[maebeale]

🤖 suggested review level: 5 Inspect 🔬 new join table + model, both edit forms reworked, decorator/importer/sort changes

A story can now reference multiple workshops and vice versa. Each linked to the workshop curriculum or typed in for one that isn't listed — and the relationship is editable from both sides. Facilitators couldn't previously connect a story to more than one workshop, or see a workshop's stories from the curriculum.

We've left in that initial relationship in order to support a data migration to retain existing connections.

What you can do now

• From a story: add any number of workshops — pick one from the curriculum, or type a free-text name, per row.
• From a workshop: connect any number of stories (each with an optional "workshop name the story used"), and the workshop page lists the stories that use it.
• "New story" from a workshop pre-selects that workshop in the form.

How it's modeled

• Join model StoryWorkshop on table stories_workshops (has_many :through both directions). Each row carries an optional workshop_id and/or an optional external_workshop_title, plus position. A row with both renders "Name (from the Workshop)".
• Migration is schema-only, no data backfill (per request). The legacy stories.workshop_id / external_workshop_title columns are kept as a safety net; display falls back to them when a story has no join rows yet, and the story edit form shows them read-only ("Previously linked") so a persisted story's pre-join workshop data never silently hides.
• A unique index on stories_workshops [story_id, workshop_id] backs the model's uniqueness validation (a concurrent double-submit can't slip a duplicate past it). External-title-only rows have a NULL workshop_id, which MySQL exempts, so a story can still carry several unlisted-workshop rows.

Notes for the reviewer

• The old single-workshop picker is replaced by cocoon nested rows on both forms. Each row's picker is a remote-select typeahead (TomSelect) that searches on demand — workshops and stories are both unbounded, so neither collection is loaded up front. This adds Story to the remote_searchable set + search/:model allowlist and a StoryPolicy#search? (authenticated) rule.
• Index "workshop" sort orders by a story's alphabetically-first linked workshop (subquery).
• Naming overlap on the workshop page: the existing "Related stories" tab shows facilitator-spotlight Resources; the new "Stories" tab shows real Story records. Kept both.

Testing

• Model spec for StoryWorkshop (needs a workshop or a title; no duplicate workshop per story).
• Request specs: workshop show lists linked stories (Turbo-Frame, incl. nil-association record); "New story" pre-selects; promote copies the idea's workshop + external title.

Follow-up

• Story ideas get the same many-to-many + the importer upsert — in a stacked PR (HOLD: Link stories and workshops as a many-to-many (both sides, + story ideas) #2586).

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