Add single-taxonomy endpoints for CRUD parity

[MA2153]

Problem

Taxonomy terms have full CRUD, but the taxonomy definition itself only
supports list + create. There's no way to read, update, or delete one taxonomy
— every other resource (menus, content, bylines) has a single-resource
endpoint. Definitions live in _emdash_taxonomy_defs; the list/create handlers
already target it, so this is purely filling in the missing operations.

Endpoints to add

1. packages/core/src/astro/routes/api/taxonomies/[name].ts

The missing single-resource route, mirroring menus/[name].ts:

Method	Perm	Handler
GET	taxonomies:read	handleTaxonomyGet (new)
PUT	taxonomies:manage	handleTaxonomyUpdate (new)
DELETE	taxonomies:manage	handleTaxonomyDelete (new)

2. packages/core/src/astro/routes/api/taxonomies/[name]/translations.ts

Mirrors menus/[name]/translations.ts:

Method	Perm	Handler
GET	taxonomies:read	handleTaxonomyDefTranslations — already exists, unexposed

POST (create-translation) is optional — translation creation already flows
through POST /taxonomies with translationOf, so it's not a parity gap.

Handlers to add (packages/core/src/api/handlers/taxonomies.ts)

All standalone, db-first, returning ApiResult<T>, following the term-level
equivalents as templates:

• handleTaxonomyGet(db, name, { locale? }) → looks up
  _emdash_taxonomy_defs by name (+locale), runs the same orphan-collection
  filtering as handleTaxonomyList, returns { taxonomy } or NOT_FOUND.
• handleTaxonomyUpdate(db, name, input) → updates label,
  labelSingular, hierarchical, collections (reuse the create handler's
  collection-validation block). Disallow renaming name for now
  (additive-only). Returns { taxonomy }.
• handleTaxonomyDelete(db, name, { locale? }) → deletes the def row and
  cascades to its terms + their content_taxonomies links.

Delete semantics

Matches collection-delete (already destructive and accepted): remove the def,
all terms in taxonomies for that taxonomy, and their content_taxonomies
rows. The term-level cleanup pattern already exists in
TaxonomyRepository.delete (database/repositories/taxonomy.ts:195) — delete
loops over the taxonomy's terms or does a bulk delete by translation_group.
Return { deleted: true }. No "non-empty" guard.

Compatibility & process

• Purely additive — three new handlers, two new route files, one
  previously-orphaned handler wired up. No schema or contract changes.
• Standard route boilerplate: export const prerender = false;, requirePerm,
  unwrapResult, parseBody + zod for PUT. CSRF header enforced by
  middleware.
• Needs a changeset (packages/core is published).
• TDD: handler-level tests for get/update/delete + the cascade, plus a route
  test that DELETE removes def, terms, and content links.

[ascorbic]

This makes sense