Revive blog posts as a content-typed gitsheets sheet (revises the deferred decision)

先阅读 issue #44 和 specs/deferred.md 中推迟的决定，然后检查现有的 gitsheet 模式和 apps/api/scripts/import-laddr.ts。跟踪 apps/api/scripts/import-laddr/translators.ts，以及所引用的 BlogPost schema 和 routing 要求。完成的标准是：在不包含已排除 workflows 的情况下，实现 content-typed blog-post sheet、API、SPA routes、恢复 importer，并完成列出的 spec 更新。

specs/deferred.md currently says blog posts get replaced by "staff-authored markdown files in the code repo at apps/web/src/content/blog/<slug>.md, shipped via PR." That decision predates gitsheets v1.2's content-typed records.

With v1.2 we can give blog posts their own gitsheets sheet — markdown bodies + TOML frontmatter — and get a better outcome than files-in-code-repo:

Why this beats the original deferral

Sheet shape

# .gitsheets/blog-posts.toml
[gitsheet]
root = 'blog-posts'
path = '${{ slug }}'

[gitsheet.format]
type = 'markdown'
body = 'body'

[gitsheet.schema]
$ref = './schemas/BlogPost.schema.json'

BlogPost entity (in packages/shared/src/schemas/blog-post.ts):

• id UUIDv7
• legacyId (laddr's BlogPost.ID, for the importer's idempotence)
• slug (kebab-case, slug-handle conventions)
• title
• summary (short markdown — stays in frontmatter)
• authorId → Person
• postedAt (iso8601)
• editedAt nullable
• featuredImageKey nullable (attachment via gitsheets)
• deletedAt nullable (soft-delete)
• body (the markdown body — the designated content field)
• standard createdAt / updatedAt

Routing

Add to the SPA:

• /blog — index (paginated, optional tag filter)
• /blog/:slug — detail
• /blog/tag/:namespace/:slug — tag-filtered (reuse TagsNamespace pattern)

API:

• GET /api/blog-posts (list with facets, q, sort, page)
• GET /api/blog-posts/:slug (detail)
• POST/PATCH/DELETE — staff-only (per the original spec, blog wasn't a per-user-role CMS)

laddr-import revival

The existing one-shot importer at apps/api/scripts/import-laddr.ts currently skips blog_posts. Re-add it as another translator in apps/api/scripts/import-laddr/translators.ts:

• Map BlogPost.Slug → slug (slugify-with-dedupe if invalid)
• Map BlogPost.Title → title
• Map BlogPost.Body → body
• Map BlogPost.AuthorID → resolve via the existing idMaps.personByLegacy
• Map BlogPost.Published (and similar) → postedAt
• Preserve legacyId so re-runs are idempotent

Sequencing

• Depends on #44 (content-typed gitsheets is the substrate) — or stand on its own as the first content-typed sheet in the project. Either order works since blog-posts is a brand-new sheet that doesn't conflict with the existing TOML-only ones.
• Sequenced after cutover-prep so existing migration paths stay valid through cutover.

Spec updates needed

• specs/deferred.md — update the "Blog (/blog) as a user-facing CMS" entry from "files in code repo" to "content-typed sheet, see this issue."
• New spec files: specs/api/blog.md, specs/screens/blog-index.md, specs/screens/blog-detail.md.
• specs/data-model.md — add BlogPost entity.
• specs/behaviors/legacy-id-mapping.md — note the new BlogPost.legacyId axis.

Out of scope: comments, reactions, the multi-author "posts under a topic" workflow — keep it as simple as the original deferral imagined.