feat: neuroglancer view (1/6): data model — views + view_layers tables

[allison-truhlar]

Neuroglancer Views — PR 1 of 6 (stacked)

Foundation of the Neuroglancer Views feature: a Neuroglancer View becomes a
first-class saved object (NG state + the datasets it shows + sharing settings).
This PR is the bottom of a 6-PR stack and is pure-additive — new tables,
models, and DB helpers only. No routes, no behavior change.

Design spec and plan (included in this PR):

• docs/superpowers/specs/2026-08-07-neuroglancer-views-design.md
• docs/superpowers/plans/2026-08-07-ngviews-01-model.md

(Scope note: this stack ships read-only Views; editable Views are a later
stack. edit_key is reserved in the schema now so that stack needs no second
migration.)

What's in this PR

• views table (ViewDB): short_key, read_key, edit_key (reserved,
  unused), name, ng_state (JSON), sharing_mode (private|read),
  owner, timestamps.
• view_layers table (ViewLayerDB): the many-to-many join between a View
  and a Data Link (proxied_paths). data_link_id is nullable and each row has
  a broken flag, so a future "mark broken" Data-Link deletion can null the
  link without deleting the View.
• DB helpers (mirroring the existing neuroglancer_states helpers):
  create_view, get_view_by_short_key, get_view_by_read_key, get_views,
  update_view, delete_view (cascades to layers), and
  get_views_for_data_link (dependent-Views lookup, powers the delete guard and
  the "Appears in N Views" discovery in later PRs).
• Pydantic models View / ViewLayer / ViewResponse. View deliberately
  does not expose edit_key.
• Alembic migration 1e8dc304b4f2 creating both tables (FK-safe, reversible),
  chained off the current head e7b2a9c4f130.

Testing

pixi run -e test test-backend → 777 passed. New tests cover the
create/get/list/update round-trip, owner-scoping on update/delete, the cascade
delete (asserts join rows are removed), the get_views_for_data_link DISTINCT
path (two layers → one View), and the Pydantic from-ORM round-trip incl.
edit_key being absent.

Stack

1. This PR — data model
2. API (Views CRUD, /ngview/{key} read link, dependent-views endpoint, Data-Link delete modes)
3. Multi-select in the file browser
4. Views page (Saved Views + Layer Cart, replaces NG Links)
5. Browser entry points (row menu, selection bar, cart rail/drawer, consent)
6. Read-only embedded viewer